Feature academy

Changes:
- Created new class CodeBadges::Academy
- Moved responsibility to get multiple users information to Academy class (get_cadets method)

Reason:
- This change is to conform to the [Interface Segregation Principle](http://en.wikipedia.org/wiki/Interface_segregation_principle) of SOLID design.
  - CodeBadges::Academy class can now have new methods for grading all students in a course
  - Developers who are not interested in multiple users do not have to use class Academy
  - New methods that compare multiple users do not have to be in the same place as those to check one user

If we agree on this change, we will have a new major version ( `_.#._` )

Please respond to this issue to discuss!