Don't override previous_tab and indent_backwards shortcuts on OSX

Prior to this commit, the shortcuts for these `paredit_wrap_square` and `paredit_wrap_curly` were overriding the OS X shortcuts for the `previous_tab` and `indent_backwards` commands built into Sublime.

The addition of `ctrl+` definitely makes these two shortcuts more cumbersome, but I think it's best not to override two of the most used built-in Sublime shortcuts.

This also disables Paredit's `ctrl+super+f` shortcut, which conflicts with Sublime's fullscreen shortcut on OS X. As far as I can tell, `paredit_forward` and `paredit_backward` mimic functionality that is already built into OS X and available almost globally via `ctrl+option+f` and `ctrl+option+b`. Should we maybe disable `paredit_backward` in the OS X keyboard defaults, as well? 

Happy to change this to another set shortcut if you have a better idea :dancer: