Rewrote course recent_activity methods

- Using single method with parameter to specify activity type
- Added modules with accepted activity types and demographics
- Converting course_id to unicode in Course constructor (to un-offically support OpaqueKey)
- Removing trailing slash from Client base_url

Change-Id: Ic21a381380fde67993b314fcf3a4529a1cf0a85b
Merge pull request #3 from edx/recent-activity

Rewrote course recent_activity methods