add getactivity API support

- add WithingsObject class that automatically converts dictionaries into
  objects with attributes, and parses dates
- refactor WithingsMeasureGroup to inherit from WithingsObject
- add WithingsActivity class which inherits from WithingsObject
- add support for inserting a version string (e.g. v2) into the API url
- add get_activities method that makes a call to the getactivity Withings
  API and returns any activities as a list of WithingsActivity objects
merge getactivity, fix conflicts