Meterpreter extension archetype

This should help anybody create new Java Meterpreter Extensions and
configuring the build system to properly build, test and deploy them.

The generated sample extension already contains two simple commands, to
serve as a base for more complex commands.


This is mainly interesting for anybody trying to contribute their own Meterpreter extensions (which before was some PITA and involved lots of trial&error even for me to build this archetype).

The generated binaries should not change, so no recompile needed.

Probably ask @mubix if this commit can close (or at least move closer to closed) [Redmine #6105](http://dev.metasploit.com/redmine/issues/6105)