Move Eliom_service.external_{,post_}service to Eliom_service.{Http,Ocaml,Unsafe}

I think ```external_service``` and ```external_post_service``` should be in ```Eliom_service.{Http,Ocaml,Unsafe}``` as the last type parameter of ```Eliom_service.service``` is polymorphic.

As a result of #56, it shoudn't be polymorphic. It also removes the type constraint in the testsuite.