Don't override OS X's built-in Command+` shortcut

OS X uses Command+` for switching windows inside an application [1]. If
a Sublime package binds an operation to Command+`, you cannot use the
standard OS X keyboard shortcut to switch windows inside Sublime.

To avoid this conflict, change the filename_status keyboard shortcut to
something else.

[1] http://support.apple.com/kb/ht1343
Merge pull request #3 from jasonrudolph/avoid-overriding-osx-window-switcher

Avoid overriding OS X's built-in Command+` shortcut