Add dropdown for users whose identity you can assume (for development on...

...ly)

For issue #1437 

This is pretty simple. The only thing of note is that I @ the variable assumable_users so that I don't have to keep passing locals around and accounting for development mode unnecessarily and I removed the "be admin" button.

Here's what it looks like now:

![screen shot 2014-03-12 at 11 01 40 pm](https://f.cloud.github.com/assets/100039/2406633/07fabbce-aa75-11e3-8147-87c3b746a9b1.png)

What do you think as a starting point?