chore(bower): refactor bower usage

The gulp bower task in the docs app was never actually running since it couldn't
find the bower.json file and was silently failing. Updating to a newer bower
highlighted this issue.

This commit moves the docs app specific bower components into the docs folder.
There are only jquery and closure compiler related components in the project
folder now.

It also improves the gulp bower task to provide better feedback of progress
and errors.