Add file upload resource

#56

![postman-3](https://cloud.githubusercontent.com/assets/125707/3352672/5d168a64-fa54-11e3-9d0f-172489852b02.jpg)

```php
file_usage_add($file, 'restful', 'files', $file->fid);
```

Should be using a known key (like `'module'`) for these tasks. Something like:

```php
define('RESTFUL_LOCK_ID', 1);
…
file_usage_add($file, 'restful', 'module', RESTFUL_LOCK_ID);
```

Otherwise you can get into weird errors when removing files if `entity_cache` is on.