TableViewCellModule cleanup

* Slimmed down TableViewCellModule by 30%
* Re-enabled some long-disabled tests (they passed)
* Added the `style:` attribute for cells — they no longer take
arbitrary attributes in the main hash, but will `set_attributes` with
whatever is in `style:`
* Removed a failing and head-scratching web bridge RPC spec (no idea
what it was for)
Merge pull request #453 from clearsightstudio/refactor/table_view_cell_cleanup

Refactor/table view cell cleanup