No more mp6

This PR (first discussed at #230) does a lot:

1.  Removes the MP6 body tag
2.  Moves many MP6-specific styles inline to places that make sense in input.css and fields.css
3.  Leaves shared dashicons/icons styles at the bottom of input.css and fields.css (a smart future change might be to add a `.cfs-dashicons` class, to de-complicate this a bit)
4.  Converts a few image icons (namely icon-question.png) to a Dashicons equivalent
5.  Removes support for "#icon-edit" screen icons, as they have been [deprecated since 3.8](https://github.com/WordPress/WordPress/blob/cd0ba24e9583a707b0ba055f0a3d9cd0f9b36549/wp-admin/includes/deprecated.php#L1135-1148)
7.  Removes now-unneeded image files
8.  Cleans up a few styles to bring some additional consistency and resolve a few minor things that cropped up

The CSS should be functionally equivalent to the previous CSS - that is, these changes _shouldn't_ break any browsers. (It's mostly the same styles, reordered.)

Woot!

![10284529163_6d9a93a812](https://cloud.githubusercontent.com/assets/1231306/4926391/1814a0d2-6533-11e4-92a7-80f8f980a42b.jpg)