Basic template works

BUG=None

Review URL: https://codereview.appspot.com/7060058
Input element bindings

BUG=None

Review URL: https://codereview.appspot.com/7066064
More element binding tests

I copied all the tests from element_bindings_test.html to
new_element_bindings_test.html and removed the bind attribute ones and fixed all
the remaining failing tests.

BUG=None

Review URL: https://codereview.appspot.com/7070067
Add rest of the template element tests

This required a few changes:

1. cloning template elements requires special handling to also clone the content.
2. When cloning and initializing the content, recursively decorate the template elements.
3. Stop observing when we remove elements

BUG=None

Review URL: https://codereview.appspot.com/7064069