Javapayload / Java Meterpreter build process refactoring

As discussed with **egypt**, this is my proposal of making Javapayload and Java Meterpreter easier to build and work with in an IDE.

Basically, both projects are placed under a common parent directory and the build file has been migrated to Maven. IDE integration has been tested with Eclipse and a bunch of unit tests have been added for the payload stager and the shell payload (meterpreter not yet).

Egypt wanted to have a look at it but now he has been for  about 180 hours idle in IRC, so I'll just send a pull request so that this is not forgotten :)

(If I should have sent the request to his github account instead, just tell me, I'll recreate it then and rebase if needed).