Close ticket on merge to master.

When a merge is made to a branch named in the config variable
'production_branch' then an attempt is made to extract the ticket ID
from the branch name. If successful then the ticket is marked as closed.
Make the default production_branch blank

This means the default behaviour is as it was before.
Add a python script to post a github-style payload.

Can be used to test the trac-hook.
Merge branch 'feature/close_on_merge_to_production'