New page: Constraints transforms

This page is only useful in the transition from the "January" to the
"April" form of GetUserMedia constraints.

It allows one to paste in a constraint in one form and see it in another.
It may also turn out to be useful as part of a polyfill later.
Addressing review comments on previous version.
Addressed review comments from May 2014
Merge branch 'constraint-transforms' of github.com:alvestrand/webrtc into constraint-transforms