Fix for issue #890

Modified the potential_mentor_count to return the number of mentors in
the project. Also, changed the test and made the result.html to work as
paulproteus has suggested. Should the function be renamed to
get_mentor_count ?
Update tests.py
1. len() has been replaced with count()
2. Number of mentors will only be printed if there is atleast one mentor
in the language
Update result.html
1. Rename potential_mentor_count to mentor_count - Done
2. "Mentors: 0 and 0 in Objective C"? - This will not be printed. The
condition has been changed to check that the mentor count will only be
printed if the project has mentors.
3. Remove the use of the values_list - Done. We now only call count on
the people in the tag query.
Merge pull request #1431 from moijes12/890patch

Fix for issue #890