Added the ability to pass matrix parameters for rest requests.

Unfortunately according to wc3 (http://www.w3.org/DesignIssues/Axioms.html#matrix) matrix parameters to http requests has been unsupported since 2001. The company I work for recently tried integrating a REST interface with third party software and their REST request required HTTP matrix parameters. Which up until this morning I had never heard of. I have added a quick patch into requests to support this.

Information on how matrix parameters are used and created is found at this link:
http://www.w3.org/DesignIssues/MatrixURIs.html