Prefix NSArray categories.

Begin process of prefixing categories on Foundation and UIKit. This
follows Cocoa best practices. This makes sense given the number of third
party libraries the project incorporates, which may include their own
colliding category method names.

Use OWS prefix on classes that may be used across apps.
Updated Travis configuration and categories guidelines.

Travis upgraded the build system including xctool.
http://blog.travis-ci.com/2014-11-03-xcode-61-beta/

Guidelines additions and Travis fix.