Removed deprecated simplejson in students.views.
As the Django 1.5-1.6 release notes recommend,
replaced it with python's json library
Small PEP8 fix in students.models
Replaced User clean() with a validator
Fixed small PEP8 mistake in User's model
Added github and linkedin validators
Style updates for show_certificate.html
Changed show_certificate's name link to github.
A link to the CourseAssignment won't be useful for non-logged users
Merge pull request #13 from HackBulgaria/dev

Added validators and updated certificates' styles