Add a trigger for comment containing a specified RegEx

This patch add a trigger that will trigger jenkins build on comment containing
a specified regular expression.

This can be usefull for user to specify a comment to run a build (eg.  Please test this )
Revert accidentally changed log level

Previous change changed trace log level to info. Revert it.
Fix topic rule for empty topic

When a topic is empty, topic regular expression never match even if
regular expression is '.*'.

Null topic are now considered a empty topic string.