Recursive binder

Very small addition to enable easy binding of recursive data structures(eg: treeviews). Here is a demo: http://jsfiddle.net/vbPrP/4/

This adds a new default binder, the 'template' binder which will use the value as a new rivets template that will be recursively bound to the child element

I have no idea why, when running the tests on CLI(phantomjs) it fails, but when running on a browser directly is passes.

I noticed that when the recursion level is too deep(in the jsfiddle just keep nesting with the 'add node' button) it gets very slow,though I'm not sure the reason.