Added ability to get activities

[thetomcraig]

Hi there! I just added some functionality for the api call "Measure - Get Activity Measures" (http://oauth.withings.com/api/doc#api-Measure-get_activity)

I had to change some logic around in init because it requires "/v2" in the GET call, so some other methods were altered.

Made a new class "WithingsActivitiesGroup" which should be pretty extensible for anyone wanting to take advantage for the activity data.

Example usage:

client = WithingsApi(creds) activities = client.get_activities(startdateymd='2015-07-30', enddateymd='2015-07-30') print "Your steps are: %s" % activities[0].steps