moved Web IDL development to GitHub

I've moved Web IDL development to GitHub, in response to a number of 
people asking for this.  Hopefully this will make it easier for people 
to contribute changes to the specification.

The GitHub repository is here:

   https://github.com/heycam/webidl

and the spec can be viewed here:

   http://heycam.github.io/webidl/

I've updated http://dev.w3.org/2006/webapi/WebIDL/ to point to the new 
location.

This revision history has been preserved, although I did need to rename 
the main spec file from Overview.{ht,x}ml to index.{ht,x}ml to get it to 
show up on GitHub pages, and the commit log viewer on GitHub doesn't 
seem to follow the rename.