Add Futurepool arg to ServiceToClient

If you want to have your Array[Byte] -> Thrift
deserialization to happen on a FuturePool, maybe
because it is very large or nested or something,
you can now pass in a FuturePool argument.

The existing 2-arg constructor was preserved so that
Thrift.newIface continues to work.
Merge pull request #47 from slackhappy/johng_futurepooldeser

Add Futurepool arg to ServiceToClient