Added Visual Studio 2013 support to node

I added building with Microsoft Visual Studio 2013 to Node.

**Note**: In order for this to be implemented, gyp needs to be updated to [r1676](https://code.google.com/p/gyp/source/detail?r=1676) or higher (I used r1743 to test)

This is a three step addition:

1. Bump gyp to r1676 or higher to get VS2013 support (Not complete, needs to be done by someone with commit access)
2. Edit vcbuild.bat to look for VS2013
3. Add a reference to VS2013 support in README.md


Results:
Vcbuild test passed with no errors and the resulting node.exe was functional.

Discussion:

The current behavior of vcbuild is to select the greatest version of visual studio possible. Is this desirable? Right now there is no way to select the visual studio version that you use. I am planning on adding a optional command argument to select what version of visual studio vcbuild looks for. However, it seemed out of scope for this pull request, so I will create a new pull request after this  one is merged. Thanks for all of the great work on node!

Other:
As this is my first time committing, I should probably mention that I signed the CLA.