Provide Base URI for shorter API calls

Hi Jonathan, my name's Darius, I'm an API Evangelist with the OPEN group here at Akamai. Thanks so much for contributing a Node.js library, this looks _great_.

I did have one bit of feedback, hence this pull request. Generally I find it's best practice to be able to provide a separate base URI for an API. This allows, for example, a developer to switch between production and development applications. It also makes for less unwieldy API calls by letting you pass in just the partial path of the API call you're trying to make.

This also makes the Node implementation of EdgeGrid more in line with the Ruby implementation. I considered adding backwards compatibility (allowing a full URL to passed in via a `url` parameter on the request object) but I'd rather enforce this standard.

I'd also love to chat with you a little about how we might promote this library (forking it to our [Akamai OPEN repo](https://github.com/akamai-open), linking to it from our developer community, that kind of thing). Please email me at dkazemi@akamai.com when you get a chance!