Implement nick coloration

This method hashes the nicks and deducts a nick color from it. Since we are using a hash and not a random function, colors will be consistent between reboots.  

The user's nickname color is always the same.

Possible improvements
 - Make our own colors, not Textual's
 - Store a cache of the hashes. I'm not sure that there is a performance hit with the current code but it might be better.  


PS : If you are interested in changing the default CIRC theme, I also ported Textual's colors to circ's CSS. It looks pretty good on a Mac, not so sure on other platforms.