Upgrade to Dispatch 0.10 & cross-compile to Scala 2.9.3, 2.10

(This is an updated version of #14, which GitHub automatically closed because I renamed the branch.)

In Dispatch 0.10, the `Promise` interface is deprecated, and `scala.concurrent.Future` is supported. 

This patch removes `Promise` support from the Content API Client library, and drops support for Scala 2.9.2. It supports Scala 2.9.3, which contains a back-ported version of the `Future` API from the 2.10 series.

In light of this being a fairly significant break in compatibility (with Dispatch & Scala versions, and the Promise API), this patch bumps the version to 2.0.0-SNAPSHOT.