Rename API.config[:handler] to API.config[:endpoint]

"handler" is a little too vague and easily confused with the
endpoint handler block.

Signed-off-by: Evan Owen <kainosnoema@gmail.com>
Rename Endpoint.handle to Endpoint.respond

Just a bit friendlier, especially when subclassing:

    class UserEndpoint < Crepe::Endpoint
      respond { User.all }
    end

Signed-off-by: Evan Owen <kainosnoema@gmail.com>