Add possibility to use a specific hostname

I'd like to add the possibility to listen to a specific hostname or IP address. This listens to the environment variable HOST and keeps it empty by default (so by default it works as before) but by assigning localhost to HOST it only listens to localhost.
Running go fmt on HOST code
Adding HOST in the readme
JMS #109: First pass at named routes
Merge pull request #160 from codegangsta/JMS-109

JMS #109: Adding named routes
Removing extra invoke
Merge remote-tracking branch 'upstream/master'