Add context menu (Fixes GH-4)

![menu](https://f.cloud.github.com/assets/1459603/2279534/9b4a7fe0-9f71-11e3-8fe1-607ed1970a85.gif)

Changes:
  - The tab click event is now a mousedown event, which gives the ability to detect a right click, and the tab will display just slightly faster now. (This is how ST does it)
  - The context menu adds "Close Tab", "Close Other Tabs", and "Close Tabs to the Right"