Proper Go packaging.

The package name is now `irc`, and the `main.go` file has been moved to
`example/bot.go` and imports `github.com/scjudd/irc`.
Fixed handled message logging bug.

If a message was handled more than once, it would have been logged more
than once.
Made handler functions return a bool.

This bool represents whether or not the handler was willing to handle a
given message. Right now, this is purely for logging purposes.
Added simple logging enable/disable interface.