Graphite web interface bind_address now configurable

This pulls in some changes proposed in #23, changes things slightly to
use the `params.pp` class, adds tests and puppetdoc.

I've also locked the rspec version at <3.0.0 because the 3.0.0 version
has broken rspec-puppet and that's what I get installed when I try to
bundle this repo. See rodjek/rspec-puppet#198

Closes #23.