Windows Universal Apps support and improvements

master.cfg - 
* moved shared steps to base class (PlatformTestBase), other platforms inherit base steps and can override them when required

Windows - 
* special logic to test Store80, Store81 and Phone81 build targets
* additional slave to test windows platform with restricted functionality (windows8, msbuild4)
* moved duplicate functionality to src/build/makers/testRunner.js
* bugfixes

Other improvements - 
* checkout.js now supports branches and custom repos