[Issue #733] Attribute Routing: Implement Order

1. Added an Order property to IRouteTemplateProvider, ReflectedAttributeRouteModel,
   AttributeRouteInfo, AttributeRouteLinkGenerationEntry and AttributeRouteMatchingEntry.

2. Changed the implementation of AttributeRoute to take the order into account when routing
   incomming requests and generating links.

3. Ensured a stable ordering of route entries with the same order and precedence for route
   matching and link generation based on the template text.

4. Added tests to validate that the precedence gets respected in route matching and link generation.

5. Added tests to validate that the order gets respected in route matching and link generation.

6. Added tests to validate that the order gets respected over the precedence for route matching
   and link generation.

7. Added tests to validate that routes with the same order and precedence expose a stable ordering
   for route matching and link generation.
I've added an explanatory comment on IRouteTemplate.Order about the usage of null.