bugzilla: add XMLRPC interface. Add support for access to Bugzilla via the XMLRPC interface. This requires a single username and password used to log in to Bugzilla, plus the URL of the Bugzilla installation. Commit messages are added to bugs as before, but security only permits them to be added as the username used to log in.

bugzilla: localise all MySQL direct access inside access class. Prepare for the addition of other Bugzilla access methods by localising direct MySQL database access inside an access class. Provide a base access class largely to document the methods required for a class implementing a particular access method. Rename the 'bugzilla_<version>' classes to 'bzmysql_<version>' to emphasise that they are doing access via direct manipulation of a MySQL database.

bugzilla: keep bug IDs in set. Bug IDs are collected into a set, and then silently converted in filter_real_bug_ids() into a list. For consistency, keep them in a set throughout and update the docstrings to say that.