Use Dispatch

I spent some time trying to debug the error we were seeing with the client due to thread safety issues but wasn't able to isolate what class was causing it:

```
2013-10-09 14:01:31,507 [play-akka.actor.default-dispatcher-2] ERROR akka.dispatch.TaskInvocation - java.lang.IllegalArgumentException: invalid version format: <U+FFF0>ￒￄ.(<U+FFFB>+ﾖYA"ￎN<U+FFBF>ﾀﾅ[ﾮD+^U^C^A^@^VXﾭ+^T|=9ﾧ%)S
java.util.concurrent.ExecutionException: java.lang.IllegalArgumentException: invalid version format: <U+FFF0>ￒￄ.(<U+FFFB>+ﾖYA"ￎN<U+FFBF>ﾀﾅ[ﾮD+^U^C^A^@^VXﾭ+^T|=9ﾧ%)S
        at com.ning.http.client.providers.netty.NettyResponseFuture.abort(NettyResponseFuture.java:324) ~[mobile-stats.jar:na]
        at com.ning.http.client.providers.netty.NettyAsyncHttpProvider.abort(NettyAsyncHttpProvider.java:1320) ~[mobile-stats.jar:na]
        at com.ning.http.client.providers.netty.NettyAsyncHttpProvider.exceptionCaught(NettyAsyncHttpProvider.java:1535) ~[mobile-stats.jar:na]
        at org.jboss.netty.channel.SimpleChannelUpstreamHandler.handleUpstream(SimpleChannelUpstreamHandler.java:112) ~[mobile-stats.jar:na]
Caused by: java.lang.IllegalArgumentException: invalid version format: <U+FFF0>ￒￄ.(<U+FFFB>+ﾖYA"ￎN<U+FFBF>ﾀﾅ[ﾮD+^U^C^A^@^VXﾭ+^T|=9ﾧ%)S
        at org.jboss.netty.handler.codec.http.HttpVersion.<init>(HttpVersion.java:102) ~[mobile-stats.jar:na]
        at org.jboss.netty.handler.codec.http.HttpVersion.valueOf(HttpVersion.java:62) ~[mobile-stats.jar:na]
        at org.jboss.netty.handler.codec.http.HttpResponseDecoder.createMessage(HttpResponseDecoder.java:104) ~[mobile-stats.jar:na]
        at org.jboss.netty.handler.codec.http.HttpMessageDecoder.decode(HttpMessageDecoder.java:189) ~[mobile-stats.jar:na]
```

This pull request replaces the direct use of Async HTTP Client with Dispatch reboot, which returns Scala Futures. I'm unsure as to whether this hinders Java interoperability.