Fixing Teacup integration with PM::TableScreen

* Fixes issue #172
* Also adds `editing_style:` attribute (can be :delete, :insert, or :none) to table cells.
* Avoids re-initializing the background UIView every time
* Better cell identifiers that uses the cell class name
* No more overwriting of `cell_style:`
* Cleaner/faster accessory initialization (avoids unnecessarily creating an accessory hash when not given)
* Minor code format fixes