added equery for Rex::Pkg::Gentoo in get_installed

Since epm is hardmasked, it's not very likely that most Gentoo users will have it installed.

equery comes from gentoolkit, and is much more likely to be installed, and can generate equivalent output to epm -qa very easily, so I've added a can_run('equery') check to get_installed to toggle behavior.
Merge remote-tracking branch 'tianon/patch-1' into 0.42