harold: Remove alarm plugin.

It's better to just use cron to run periodic harold tasks.
harold: Remove ident plugin.

It was just a silly hack to make connecting to freenode faster.
irc: Make HTTP plugin an optional dependency.
plugins: Add a more descriptive error for unsatisfied dependencies.
alerts: Allow Jabber and SMTP to be optional.
alerts: Rename alert expiration time to "ttl".
Reorganize documentation.