Add update all button

Shows all the available updates with an update all button in the Packages section.

![update-all](https://cloud.githubusercontent.com/assets/671378/2823763/4b851bae-cf24-11e3-856b-3f0515b7ff54.gif)

- [x] Add available updates section to Packages panel
- [x] Add status bar icon that display number of available updates
- [x] Update [apm](https://github.com/atom/apm) in core that includes the new `apm outdated` command.

Closes #49
Closes #114