add fun-certificates app containing the module generator.py and the command generate_fun_certificates.py
remove useless loop in COURSE_LISTINGS
#648 PEP8 corrections
#648 add reportlab requirement, used in the generation of certificates.
#599 Add option set-grade to add external grade
#648 test existance of 'grade' option before converting it to float
#648 also generate the certificate when we set the external grade
#648 do not filter students that haven't a profile
Merge pull request #7 from openfun/#648-certificates-generation

#648 FUN certificates generation