Implement a RFC 6570 compliant UriTemplate

[justinfagnani]

It seems like it might a good idea to use a standard URI template syntax rather than one of the many ad-hoc syntaxes, including route's UriPattern.

http://tools.ietf.org/html/rfc6570

The spec is focused on URI expansion and vague about variable extraction, so we'll have to add some validation to ensure templates are reversible.