Gentoo support

Those commits bring proper Gentoo support to the module, through keywords, useflags and dependencies. I splitted package.pp to package/{agent,server}.pp in order to make the separation cleaner. There are more changes here and there as well.
I tested against Gentoo (both Puppet 2.7 and 3.1, and both Ruby 1.8 and 1.9), openSUSE 12.1 and SLES