Vs2012/2013 support

Hi there,

Apologies - I'm pretty new to github, so not 100% with the correct etiquette for contributing to someone else's project.

ANYHOW - I was playing with Emscripten over the weekend, and made a few changes to vs-tool to support Visual Studio 2013. The changes are mostly taken from spotting 'what VS-Android did', so I'm not going to pretend I've a deep understanding for the reasoning behind them all. The emscripten integration does, from what I can tell, now work in VS2012 and VS2013. I don't have VS2010 however, so you might want to check that's not been broken....

Summary of changes: 
* Switched out the vs-android cpp tasks dll for the three different platform specific dlls 
* Made MSBuild changes (mostly copied from VS-Android, searching for 'VisualStudioVersion')
* Added an 'install.cmd' to install into relevant installations with the correct DLL.

TODO:
* Check VS2010 still works....
* Change the non emscripten packages to add the VisualStudioVersion conditions - I might get round to this later, but I've no pressing need myself to do this.

Cheers,

James