OEC-486, rake oec:courses requires clean directory; no dep on timestamp

https://jira.ets.berkeley.edu/jira/browse/OEC-486
- Renamed CoursesWrapper to be more specific
- Timestamp no longer a part of export directory / file naming
- Running 'rake oec:courses' will fail if target dir is not empty. The timestamp usage was not serving Ops and Justin well.

Also, with this commit I verified production bug https://jira.ets.berkeley.edu/jira/browse/OEC-485 is not reproducible.