Upgrade to Dispatch 0.10

This patch upgrades to version 0.10 of the Dispatch library.

In Dispatch 0.10, the `Promise` interface is deprecated, and `scala.concurrent.Future` is supported.

This patch does not completely remove `Promise` from the client API, but this can be done in a separate patch. @robertberry is working on a branch of Mobile Aggregator using Dispatch 0.10, and AFAIK is currently still using `Promise`.

This patch can be back-ported to Scala 2.9.3, which contains a back-ported version of `scala.concurrent.Future` and thus can support Dispatch 0.10.