add support to select patch sets

- add new editable column to select the patch set to be displayed for each change in the changes list
- all actions on the changes list row are now based on the selected patch set (review, cherry pick, etc)
- display changes for the selected patch set in the changes browser
- display and add comments for the selected patch set in diff view
- resolves #22

- I did not update information in the list panel based on the selection of a patch set (such as the subject or review labels). I think this would be rather confusing... Since the commit message is visible in the changes browser now and changes to the message can be viewed there, I don't think this would be necessary. The labels are also not really of any interest, since labels are only considered for the latest patch set.
- Also, it is not really visible, that the revision column can be selected. I don't know if there's a good way to make this a little bit more apparent, so that users are able to find the feature without reading a manual...

Screenshots:
![screen shot 2014-06-18 at 02 09 34 pm](https://cloud.githubusercontent.com/assets/3630295/3320631/f1d82a0c-f72c-11e3-98f9-8c8f7bbe989e.png)
![screen shot 2014-06-18 at 02 04 04 pm](https://cloud.githubusercontent.com/assets/3630295/3320635/f5a976ae-f72c-11e3-9c72-4ac715d416f8.png)