Allow resource mapper to use multiple base urls 

Fix for GPRESOURCES-18, allow multiple base urls which are chosen off a
hash of the resource name.  This way a given resource is always resolved
to the same baseUrl.  Backwards compatible with current way of defining
a single baseUrl.
Test cases for rotating baseUrl

Tests working with present configuration, with empty list and populated
list of baseUrls, and that a resource resolves to the same baseUrl
consistently.
Merge pull request #51 from cmagnuson/develop

Allow resource mapper to use multiple base urls