* Added MySQL to SQLite migration instructions; SQLite setup instructions

* Renamed schema.sql to mysql_schema.sql

* Produced modified schema suitable for SQLite

* Added SQLite setup and migration instructions

File expiration support has never been properly implemented in Pomf core
or moe panel. It serves as legacy baggage before nekunekus got an idea
to create nokonoko/Uguu for temporary uploads.

In current plans, this non-feature will be removed in the 3.0.0 release
which is the next SemVer major version that allows to remove this.

Reviewed-by: Austin Gillmann <alucard@cuntflaps.me>

Accounts, invites and reports are not used in the core. These will be
moved to pomf-moe repository. You don't need it anymore.

While the change of `orginalname` to `originalname` is good to mention,
it is better served in the documentantion for those who are upgrading
instead of laying around useless here in this code repository.

OK pantsucat

DATABASE UPDATE: `orginalname` renamed to `originalname` (run update.sql to update `files` table).
Run `update.sql` to update your database, no data will be lost (if you don't update it, pomf won't work properly).

`schema.sql`
- `id`s are now unsigned to double the maximum number of supported indexes
- `size` and `user` columns are unsigned too (we can't have a negative file size or a negative user id)

`static/classes/Response.class.php`
New class to manage responses and errors. Using a class for responses makes the code cleaner and easier to add new response type handlers in the future.
- The response type is declared while a new variable is initialized and the `error` and `send` methods will print a proper output according to it
- The header for the response is also set accordingly to the response type (`text/plain` for `csv`/`gyazo` output, `application/json` for `json` output)

`static/classes/UploadException.class.php`
`Exception` class extension to manage errors that may have occurred while uploading files to the server. This way every error is reported in detail.

`static/includes/settings.inc.php`
- `POMF_URL`: add an option to set the domain of the url to prepend to the newly generated filename (before it was hardcoded and it was `http://a.pomf.se/`) [this option is currently used only when the output is set to `gyazo`]

`upload.php`
- Add functions calls at the beginning to enable the output compression if possible (gzipped output)
- Include new classes
- `generate_name ()`
--- The new name generation algorithm is basically the same of the old one, but it's easier to add/remove characters if needed (eg. lower-case letters isn't enough and you need to add capital-case letters or digits)
--- Check directly the database instead of the filesystem to see if a generated filename is already in use
- `upload_file ()`
--- File errors generate a `UploadException` instead of a normal `Exception`
--- Move common code outside the `if` to avoid useless code repetition (only the query is now inside the condition)
--- Change file permissions of a file after moving it to the new directory (if the user didn't set a mask previouly these files wouldn't be world readable, we make sure they are)
- `diverse_array ()`
--- Function to reorder initial `$_FILES` array (easier to handle for `refiles`)
- `refiles ()`
--- Use a `foreach` cicle instead of a `for` one, easier to understand
- Create a new response object and set its response type (`csv`, `gyazo` or default to `json`)
- Edit response handling according to the new response class