Add CTCP dispatch mixin.

This mixin allows one to handle CTCP requests and responses similar to IRC
commands.  on_ctcp_$COMMAND will be called when a CTCP request is made, and
on_nctcp_$COMMAND will be called when a CTCP response is received.

This also implements a basic VERSION response, returning the current version
of the PyIRC2 library in use.
Add CTCP PING handler.
CTCP: imports to the top, _version as an attribute
Merge pull request #5 from Elizacat/ctcp

Add CTCP dispatch mixin.