WIP Add a threading back end that uses c++11's std::thread

For now it doesn't compile:
 - need to implement ThreadSpecificKey stuff
 - need to compile everything with exceptions enabled, or to compile a wrapper
   library for std::thread, std::mutex and std::condition_variable that catch
   exceptions and signals errors in a different way
Add a JavaScriptCore build for Windows Phone 8
Fix recursive mutex
Test project showing JavaScript execution in WP8 app