Opening up the option to supply a different instagram api hostname. 

Consider writing a web app that interacts with the Instagram API. It can be advantageous to write a mock Instagram API that implements some subset of the real Instagram API for things like load testing. 

I am doing exactly this. Thus, I need the ability to point instagram-node at a host other than 'api.instagram.com'

This PR opens instagram-node so as to accept any Instagram API hostname, while defaulting to 'api.instagram.com' when no override is provided.

Thanks.