Only display user first, last name if they exist

:fork_and_knife: When looking for collaborators via select2 only display the users first and last name if they have one (in Supermarket), otherwise just display their username.

From

![screen shot 2014-08-07 at 4 19 51 pm](https://cloud.githubusercontent.com/assets/316507/3848485/5c9f9978-1e70-11e4-8e13-7839f18a107e.png)

To

![screen shot 2014-08-07 at 4 20 40 pm](https://cloud.githubusercontent.com/assets/316507/3848487/5f746c82-1e70-11e4-8156-105c064c99fb.png)