course and enrolment models

this depends on #19

1. new course model

2. course user relation model: enrolment
1. 'new courses' is better :-)

2. Okay, but why new objects are states ? a bit hard for me to imagine.. any other similar states ( incase I wrote describe again next time) ? 

3. Seems most of the examples use <code>does not</code> for negative case of <code>should not</code>.