Extension manager

This is the new extension manager as started by @michitux at the hackfest. I continued work on it and brought it to a state that's usable and mergable in my opinion. Note: this is a complete rewrite and just reuses the look'n'feel of Pyush's project.

Things that should be done before merging (preferable by people other than me):

* code review. I might have done something stupid. especially security should be rechecked
* real life tests. throw shit at it and see if it breaks
* (re)add RTL styles. I threw them out to keep things easy when reevaluate the needed CSS (sorry @selfthinker)
* cross browser tests
* whatever I forgot

![start test wiki](https://f.cloud.github.com/assets/86426/943825/ec28f42c-026a-11e3-8cc3-00b98c8927f7.png)