Now nickname and channel matching is server aware: we only match words that
have the prefixes the server uses.

The url regex matching was designed to be more versatile. This was needed to
disambiguate matches between nicks and channels, since they may have common
prefixes now (&+!). In case of ambiguity, when the server supports prefixes that
are common to both channels and nicknames, we choose the nickname (the nickname
only matches if there is a user with that nick in the current session).

Fixes #655.
Indentation fix.
Fixed memory leak.
Merge pull request #700 from orium/url-chan-prefix

Server aware nickname and channel matching