AppDelegate: Style Fixes.
Git!-Info.plist: Declare that Open In is supported.

This doesn’t support *all* the plain text types that Git! needs to
support. But it’s a start.
IDTFileViewController: Style Fixes.
NSError+AlertView: Make a direct comparison.

This led to a bug where a NULL NSError was still valid after the check.
IDTRepositoryTableViewController.h: Style Fixes
Add IDTReplacementFile.

This object handles all the file operations in the open-in process.
Add open-in support, and the features surrounding it.
Add tests.
Rename files for clarity.

IDTReplacementAlertViewDelegate -> IDTReplacementSystemAlertViewDelegate
Merge pull request #18 from PiersonBro/replace-in

Add Replace in Support