Provide a Kotlin DSL for the functional Web API

It would be nice to find a way to avoid specifying explicitly HandlerFunction lambda type for Kotlin user since they won't be able to fix this for Kotlin 1.1 (see this explanation on KT-14923).

Possible solutions are :

• Provide Kotlin specific API for the router
• Provide Kotlin extensions for RouterFunction and RouterFunctions
• Modify Java API signature to avoid this issue (unlikely but this is a possible solution)