Shorten output for custom matcher

Don't show the full object.inspect, just the name of the object. This was particularly noisy with documentation output.

Goes from this:

![image](https://cloud.githubusercontent.com/assets/4521/5428412/791a0cc8-8388-11e4-88e7-d777312ea79d.png)

To this:

![image](https://cloud.githubusercontent.com/assets/4521/5428408/5b8e5830-8388-11e4-9036-f4dfe20daf2f.png)