Add --recursive option when running mocha tests

This automatically includes test files in subdirectories
Add angular-bootstrap-nav-tree as a dependency

Doing this manually for now (not through bower)
Make the abn-tree directive use a template URL instead of hardcoding the entirety of the template inline

Also, make the template URL configurable via template-url attribute. Example: <abn-tree tree-data="..." template-url="/partials/abn_tree_template.html">