Add support for importing from Google Docs

This is a replacement PR for #43 .

New entry in the Add drop down: New Google Doc Import

When that entry is clicked:

1. Google Doc picker is thrown
2. Html for selected Google Doc is fetched for google
3. Called remix's gdoc2html service which transform html into aloha-ready html
4. aloha-ready html is put into the editable area

Unresolved issues:

- [ ] Icon for New Google Doc Import is Google+ and not Google
- [ ] Using gdoc2html from http://testing.oerpub.org/ instead of http://remix.oerpub.org/ (just need to release remix)
- [ ] If the GDoc import fails (or user cancels out of the picker dialog box), we need to run some cleanup code.  release does include an empty stub _cleanupFailedImport() that needs to be filled.