Warnings as Errors off by  default

Having warnings as errors on by default has proven itself to be too unstable for general use. So this is now turned off by default. TeamCity builds continue to build with Warnings as Errors turned on though in order to keep Pull Requests clean on the TeamCity set of canonical builds. I have also updated http://www.qgroundcontrol.org/dev/contribute to reflect the GitHub Pull Request process as well as how TeamCity works. Please take a look at that page as well.

This pull also includes some warnings fixes which show up with the latest clang version.