Updated generate_certificates.py
Now it should handle correctly forked repos and commits through HTTPS.
Changed Courses.models.Certificate field.
An unique ForeignKey is just a one-to-one relationship,
that can be expressed with OneToOneField
Updated CourseAssignment's methods.
Key user was misleading. It's better to explicitly say it's a username
Added coverage and restructured requirements
Renamed files.
Updated coverage.rc with more files to omit
Updated coverage.rc, INSTALL.md and README.md
coverage.rc correctly ommits now,
INSTALL.md has updated instructions for the new requirements structure,
README.md has a coveralls.io badge