Homepage sponsor admin

# Changes
 - Allows sponsors on the homepage to be updated on the homepage node (based on Partners taxonomy).
 - Adds a new image style with expected dimensions for homepage partner images & a desaturate filter.

# Example
![screen shot 2014-10-20 at 5 33 14 pm](https://cloud.githubusercontent.com/assets/583202/4709614/b939b5b4-58a0-11e4-950d-046a2a6bcacb.png)

Entity metadata wrappers are kind of a beast I introduced into the codebase that I'm hoping we refactor enough to move away from. Mainly because if a field isn't set, the site breaks with that "Fatal exception" WSOD.

This is ok to keep for now, but can you just make sure / confirm that adding a Partner term without a logo doesn't break the homepage?