Add two ways sync with Google Drive

[marccarre]

As far as I understand (please correct me if I am wrong),

• there are ways to synchronize one's todo.txt with Google Docs/Drive from a computer/todo.txt-cli (tough this page indicates this may no longer be working), but...
• that computer/todo.txt-cli becomes the bridge between DropBox and Google Drive, and
• there is currently no way to use Google Drive as a two-ways synchronization mechanism like done with DropBox natively from todo.txt-android.

It would be great if we could

• add support for Google Drive (so that it could replace completely DropBox), and even maybe,
• have both enabled in todo.txt-android and choose the kind of synchronization desired: pull, push and two-ways.

I'm happy to provide a PR provided minimal guidance regarding:

• long term strategy for these apps,
• design (e.g. would this be a plugin? a core functionality? etc.)
• other technical details which may be relevant.

Thanks, Marc.

[evanp]

There's a superclass for remote storage, so this should be relatively easy to integrate.