Added new GetElementsById instruction, and reimplemented getElementById to use.  This prevents lots of messages between TPG and client when retrieving IDs because they can be done in a batch now.
Merge branch 'master' of git://github.com/daniel-r-austin/threepenny-gui into daniel-r-austin-master

Conflicts:
	src/Graphics/UI/Threepenny.hs
Merge branch 'refs/heads/daniel-r-austin-master' into html-combinators