Support server mods with mark2, fixes #6

This adds support for fetching any of the mark2 supported jar files. 
This is currently about 35 different jars, including craftbukkit.

Switching jar files even sort of works, but requires changing the
attribute, running chef and a manual restart of mark2. This could
probably be better handled.

Also, ```include::sudo``` in the server recipe isn't required,
unless there's a reason to assume sudo won't be installed on
one of the cookbook supported platforms.