Make base URL pluggable

Hey Rob...  Happy new year ;) 

I've been playing with the PT API and v4 is in alpha now which prompted me to make the API URL pluggable. Not sure if you favor more complication to support an argument like `pt = Pivotal(TOKEN, v=4)` or even making the URL a kwarg? But this get's the job done for me and I figure you'll just bump the base to v4 when it's officially supported anyway.