Added "regexp" command, ported from Urban Airship pmxbot at tag 1001.

The regexp command is like contains in that it watches all content,
however it allows you to monitor for a certain regexp condition and
pass a match.group() value instead of the normal message. Useful to
auto-generate links from a ticket number, for example.
Cleaning up a formatting issue.
Added README information about regexp.

Let me know if the regexp command needs more usage information,
the example seems pretty clear to me.
Fix indentation bug due to tabs vs. spaces
Moved contains matching and processing into the ContainsHandler
Implemented RegexpHandler as a subclass of ContainsHandler
Corrected syntax error in regular expression
Pass the whole match object to regexp handlers (so they have access to the whole string or other details of the matched expression if necessary).
extract much of the contains logic into the ContainsHandler (creating _channel_match and _rate_match)
Now the both ContainsHandlers and RegexpHandlers can be dispatched with the same logic
Added tag 1104.3 for changeset bfb9f88f3aea
Updated changelog
Rename files with rst extensions for better rendering.

--HG--
rename : CHANGES => CHANGES.rst
rename : README => README.rst
Moved some attributes and methods from ContainsHandler to parent Handler class
Extracted match and process into CommandHandler class
Collapsed now redundant tests when dispatching handlers
Just pass the handler object to the exception handler.
Type attributes are no longer necessary. The class is the type.
Updated README reference
Fix help to no longer use type_ attribute