[WIP] Update UI to be spiffingly spiffing

:fire::fire::fire::fire::fire::fire::fire::fire::fire::fire::fire:
:fire: NOT READY TO MERGE! :fire:
:fire::fire::fire::fire::fire::fire::fire::fire::fire::fire::fire:

Based upon my terrible, cruel, inhumane criticisms of Alcatraz's main window in #129, @orta produced some revised designs for the package manager window. This is the initial implementation of that design. 

Things I haven't implemented/need discussion:

 - [ ] I've switched to using a proper NSToolBar, and part of that was dropping the toolbar icons. I can put them back, but they may need to be updated to work at 48 x 48 px
 - [ ] The "installing" state of the install button is missing the indeterminate webby progress indicator thing from the design
 - [ ] Many of the state changes to the install button aren't animated
 - [ ] Spacing between items might be a bit wonky in places
 - [ ] There are probably things in the code that need work/fixing

Your move, @orta: 

![screen shot 2014-03-31 at 2 15 14 am](https://cloud.githubusercontent.com/assets/4802/2561560/16c5c160-b81e-11e3-934c-69432cd1877f.png)