Make the jira service version 4 compatible

Jira4 has a far older API which doesn't behave in the same way. This
commit should make both version 4 and 5 be usable. Introduced a new
boolean variable jira.version4.
Fixes for backward compatibility

Some fixes to the code so we don't break existing configuration files.
Those are somewhat discussed at https://github.com/ralphbean/bugwarrior/pull/68
Make the multiprocessing option really optional.
Merge pull request #68 from nikolavp/jira4-fixes

Make the jira service version 4 compatible
Support filtering by repo for github.  Fixes #72.
Merge branch 'develop' of https://github.com/ralphbean/bugwarrior into develop