Add transfer backend for Google Storage

[Jason Gerard DeRose]

So dmedia has a very abstract concept of a "store". A store can be the native dmedia FileStore somewhere on the local file system, or it can be a remote service like S3 or whatever.

So we should of course support Google Storage: http://code.google.com/apis/storage/

Currently S3 (via boto) is the only proper remote store supported (both upload and download). You can see the S3 backend plugin here:

http://bazaar.launchpad.net/~dmedia/dmedia/trunk/view/head:/dmedia/backends/s3.py#L97

The base class may not be generic enough yet, may not have the right abstractions:

http://bazaar.launchpad.net/~dmedia/dmedia/trunk/view/head:/dmedia/transfers.py#L243

So this bug may take some slight changes to the TransferBackend base class.

Also see the related bug to add a transfer backend for the UbuntuOne Files API:

https://bugs.launchpad.net/dmedia/+bug/830684

And the related bug to add a transfer backend for OpenStack Swift:

https://bugs.launchpad.net/dmedia/+bug/830687