Added modal window feature

Added  a slide up from the bottom modal window that could be used to: 

* Choose the account that you're posting with (see Tumblr.app) 
* Confirm the details of a form before posting it
* Post a reply to a message thread

![modal-demo](https://f.cloud.github.com/assets/924/29351/e0a1a3b0-4d61-11e2-8b6f-8ac3cbe93b91.gif)


* See `test/components/modals.html` for a working demo. 
* Tested in webkit, only. 
* Uses position: fixed, so it won't work in iOS before version 5.0

Any queries? @fat @dhgamache @connors ?



Yes, that *is* tribe.