redmine_interface.py: add support for additional trackers

This commit add support for multiple trackers
(ie ticket categories) through the use of the redmine section
in the bot's configuration file.
It works by reading the 'trackers' section of the config file
and dynamically mapping the confgiured trackers to those detected
by querying the redmine server (that is also configured in the bot's
config file).

The sample config file has been updated with an example
of how to configure the use of trackers.

[TechnuxBot #12]