support backend commands

[musketyr]

following commands for managing backends are not implemented yet

http://code.google.com/intl/cs/appengine/docs/java/backends/overview.html#Commands

[musketyr]

there might be also some task like gaeUploadAll which will simply upload to the frontend and all backends because this is the most common use case.

[bmuschko]

I added all backends tasks to the plugin in a separate branch. Can you do me a favor and try it out before I merge it into master and release it?

[musketyr]

ok. I'll try it and let you know

[musketyr]

works like a charm. good work, thx!