Add prefixes to sf2ghJSON.py.

Add the title prefixes based on tracker names also to the sf2ghJSON.py
script. If the tracker has a non-standard name, no prefix is added (same
behavior as before). If it has a standard name, the same prefixes as in
issues.py are used.
Merge pull request #28 from legordian/master

Add prefixes to sf2ghJSON.py.