Simplify Dummybackend init
Register regex-based bot commands
Unittest current callback_message behaviour
Unittests for access controls
Detect and execute regex-based commands
Remove unused import
Allow regex commands to have overlapping regexes

This will allow messages to match multiple regex-based commands
simultaniously. When this happens, all matching commands will be
executed.
Allow passing re flags to regex commands

This allows the regex behaviour to be adjusted, such as by setting
re.IGNORECASE in order to perform case-insensitive matching.
Move decorators into a dedicated submodule

I've mainly done this because Sphinx doesn't pick up on functions inside
`errbot/__init__.py` so @botcmd and @re_botcmd were going undocumented
in the docs. This change ensures they can be referred to easily in the
docs.

While at it, the docstrings for these two decorators, as well as
@webhook, have been greatly expanded.
Adjust the error returned when lacking bot-admin privileges

I accidentally broke it bit during refactoring :)
Merge pull request #250 from zoni/re_botcmd

Regex-based bot commands