@1aurabrown => Switch View

I'm holding off on calling this "done" because of this (serious) problem with our testing suite: https://github.com/artsy/eidolon/issues/52

However, it's in an OK place to start a code review. 

Also I added in an Objective-C category taken from eigen. I'll probably swiftify it at some point. 
It's kind of moot in this specific case since the remaining width of the switch view is 894 and there are 6 buttons, which means each button gets 149 even points of width.