Javascriptcore wp8.1 cleanup

Start from https://github.com/appcelerator/webkit/commit/769be81be9d7ca1b0189fcdd9dc896d0750e975e and then add Windows and Windows Phone Visual Studio project files.

- To build JavaScriptCore for Windows Phone emulator, use JavaScriptCore-Win81.vxcproj.
- To build JavaScriptCore for Windows Phone device (ARM), use JavaScriptCore-WP81.vxcproj.