Include jacoco for code coverage.

Hello,

I understand your concerns in regards of execution time and site size.

I moved the execution of jacoco into a profile. Now by default it will not be executed. Running:

```
mvn -Pwith-jacoco clean verify (2m11s)
mvn clean verify (1m45s)
```

now will run tests with jacoco. On my machine (MacOSX 10.8.3, JDK 1.7.0_25 with a SSD) the difference between running with and without jacoco results in 2m11s versus 1m45s.