Inline variant of withTraceContext using macros.

As the title says :)

Tests pass. Traced PingPong and Function0 seems to have gone the way of the Doh-doh.
Hopefully, this should help with https://github.com/kamon-io/Kamon/issues/12

Attaching a couple of captures: First is a PingPong trace, second is the same trace using macros.

![pingpong-trace](https://cloud.githubusercontent.com/assets/6013514/3389293/2964d0a4-fc93-11e3-8762-2cf47f783849.png)
![pingpong-trace-inline](https://cloud.githubusercontent.com/assets/6013514/3389294/296504f2-fc93-11e3-8ac5-5257c24c9ad1.png)