Expose background activities

[giampaolo]

http://developer.runkeeper.com/healthgraph/background-activities
They are currently not exposed.
I would expect healthgraph.resources.User class to provide a get_background_activities() method similar to the others already available.
In my case I need background activities to retrieve the total steps taken in a day.

[giampaolo]

I just implemented this in my branch:
https://github.com/giampaolo/healthgraph-api/commit/699b6b245690b5d6a0552ab9f9f5bc106b5b1b0f
I suppose different things are missing but well... here goes.