Keep track of baseUrl root path and handle path-absolute resources.

This is related to issue #620.

API endpoint returns a path-absolute self link, such as **/api/v1/person/5**.

baseUrl setting includes part of this path, such as **http://some.host:8000/api/v1/**.
baseUrlRoot will be set to **http://some.host:8000**.

If **setSelfLinkAbsoluteUrl** is not true, then a second check will happen to see if the self link begins with a slash. If so it is treated as path-relative and appended to **baseUrlRoot**.

This covers my own issues with self links cross domain as well as the path-absolute issues touched upon in #620.