Script for provisioning a development environment.

Automates and updates the developer tools instructions that are
currently at
http://docs.swiftnav.com/wiki/HOW-TO:_Setting_up_developer_tools_for_Piksi. This
has only seriously been tested on OS X 10.9.* on my personal machine so far.

r=@fnoble,@cbeighley,@njoubert,@imh,@kovach,@gsmcmullin