enable building graphql schema using KFunction method references

[pabl0rg]

Many times a project already has Services with methods. These methods can have many arguments and it is tedious to expose them with the lambda style that is currently supported.

These changes allow using KFunction method references which include all argument information and reduce boilerplate.

For example:

val mlService = MovieLensService(MovieRepository, GenresRepository, OccupationRepository, RatingRepository, UserRepository)

val schema = KGraphQL.schema{
    query("allUsers"){
        resolver{ -> mlService.getAllUsers() }
    }

    query("getUser"){
        resolver{ id: Int -> mlService.getUser(id) }
    }

    mutation("createUser"){
        resolver{ age: Int, gender: String, occupationId: Long, zipCode: String ->  mlService.createUser(age, gender, occupationId, zipCode) }
    }
}

becomes:

val mlService = MovieLensService(MovieRepository, GenresRepository, OccupationRepository, RatingRepository, UserRepository)

val schema = KGraphQL.schema{
    query("allUsers"){
        mlService::getAllUsers.toResolver()
    }

    query("getUser"){
        mlService::getUser.toResolver()
    }

    mutation("createUser"){
        mlService::createUser.toResolver()
    }
}

[coveralls]

Coverage decreased (-0.01%) to 87.846% when pulling e8dfd02 on pabl0rg:master into 42fef4a on pgutkowski:master.