Fix contentFor directive: Do not compile already compiled content

Hey @mcasimir,

I just stumbled upon the following bug. In the contentFor directive, we use the already compiled content in the postLink function to yield (compile and link) the content for the target element.
This works for things like ngBind without problems, but ngRepeat for instance removes the ng-repeat attribute in the compile step and inserts a comment ```<!-- ng-repeat: ... -->```. When we compile that again, the ngRepeat directive is not invoked anymore.
I created a simple plunker to reproduce that: http://plnkr.co/edit/oscFV3P7mKKaVfA5u51b?p=info

I solved this issue by using the compile function, which gives us the raw contents before angular's compile step. This works in my app and doesn't break the demo, feel free to merge or copy the code.

Have a nice day,
Julian