add difftool support

This adds an option to use `git difftool` instead of the internal file comparison tool if you have a difftool configured in git.

![Git Settings with difftool option](https://cloud.githubusercontent.com/assets/250278/4141220/62a488f0-33ac-11e4-8e1b-1e346f113026.png)

For example, I prefer p4merge for file comparison.  Clicking the diff button in the git panel now opens p4merge (by calling `git difftool`) instead of the built in comparison tool.

In the history view, the expand/collapse comparison is retained, but, if enabled, a button is added to view the change using difftool.

![button added to history view](https://cloud.githubusercontent.com/assets/250278/4141367/8dc31500-33ad-11e4-90f3-7604324a4ae6.png)