fix compare to be more correct
Introduce second popular package manager for gentoo: [palugis](http://paludis.exherbo.org/).
Reorganise Gentoo package management a little. Handle dependency spec errors.
Fix missed exeception name: PackageManagerOperationalException -> NoPackageManagerOperationalException
remove redundant (for system package menagers) method
Turn a free function into a "mix-in" class to inherit by Gentoo's
package manager classes.
Move some common gentoo packagers code to the mix-in class
add missed param
Merge pull request #141 from zaufi/master

Improve Gentoo package managers support