fix RawZipkinTracer when traces.length > 10

I noticed in my testing that using node-tryfer with RawZipkinTracer
directly worked, but using ZipkinTracer with the buffering casued traces
to go missing. After some digging I discovered that
RawZipkinTracer.sendTraces is doing:

    async.forEachLimit(traces, 10, this._sendTrace.bind(this),
                      callback);

The problem is that the _sendTrace method wasn't accepting a callback,
so if there are more than 10 traces they just get stuck in limbo
forever. This patch fixes _sendTrace to use the callback that is sent to
it.
Added test for 9bd661d (wadey:fix-zipkin-sendtraces), which ensures that the callback passed to RawZipkinTracer._sendTrace is called at the end of the waterfall.
Fix a secondary problem causing traces past the first 10 to not get sent - scribe.Scribe.send is not an async function, which means it doesn't take a callback, which is required by async.waterfall.  Wrap it in a function that takes a callback, so the async.waterfall can end.
Remove async.waterfall call in RawTracerZipkin._sendTrace, since there is only one async call.
Bump the version number
Merge pull request #17 from racker/rawzipkintracer-sendtrace-limit-test

Added test and additional fix for https://github.com/racker/node-tryfer/pull/16