Unit System option

Fixes Issue #7 by adding optional unit_system parameter in client constructor. If not specified, fitbit responses will default to the metric system (as per their API design). Also changed the test to grab the profile of the current authorized user, because I found this to be a better test of the unit system switch. The old test used a fixed date for grabbing activity data, which wasn't always guaranteed to have any data. The profile should at least show some data. Also markdown referenced directory test which does not exist. Updated to say example.