Configure contracts to be run only for a fraction of method calls, ensuring a limited overhead during runtime

For example, you could configure a class ensuring that running the contracts takes at most 10% of the time it took running the actual methods of a class:

```
<class name="de.vksi.c4j.systemtest.config.contractoverhead.ContractOverheadSystemTest">
    <contract-overhead>10%</contract-overhead>
</class>
```
