Added nickserv module

In issue https://github.com/ayust/kitnirc/issues/2 there was discussion of creating a nickserv module. Here is one way of doing it. It is pretty simple but I think it is a pattern that will be repeated enough times to justify it's own module. 

## Problems 
Does not check if the authentication was successful. A possible way of doing this is parsing the notice the nickserv returns. I was not sure if the nickserv message was a standard format.