Add 2 unofficial API calls: Clean up database, and refresh.

Add 2 unofficial API calls:

### cleanupDatabase
> Same as selecting 'Settings' > 'Clean-up Database' with Subsonic web GUI
>
> Subsonic stores information about all media files ever encountered. By cleaning up the database,
> information about files that are no longer in your media collection is permanently removed.
>
> returns True if initiated successfully, False otherwise

### scanMediaFolders
> Same as selecting 'Settings' > 'Scan media folders now' with Subsonic web GUI
>
> Refreshes Subsonic media database.
>
> returns True if initiated successfully, False otherwise

        
I would rather have the library consumer handle the exception here instead of the library itself.  Returning a boolean is fine, but the standard API actually raises exceptions in the event of a failed server call.  I'd like to keep that consistency.