Test forking during release
Call Retrolamdba directly from the Maven plugin, without reflection

 We have to run Retrolambda on itself, because the Maven plugin requires Java 6
 and would not otherwise compile against Retrolambda's Java 8 classes.
Refactored
Fixed the Maven plugin not working under Java 6

- Main no longer requires Java 8 classes during its class loading
- Forces the process to fork when not under Java 8
The use of a Java agent is now optional

Closes #27