master.cfg: platform test steps via base class and overrides
improve checkout to support custom repos and branches
additional build configurations for windows
windows universal apps support
new script to write test configuration to config.xml
Updates README with actual configuration and notes. (+1 squashed commits)

Squashed commits:

[c9d38ac] updates documentation
Added back logic to update cordova repos instead of re-cloning them
Windows: logic to specify target build version via config

As per cahnges in Windows Universal Apps logic:
* no --store80 switch anymore;
* target version is specified via config.xml:
<preference name="windows-target-version" value="VERSION" />