Refactor the install.pp

Replace $pe_ruby and $use_bundle variables with $provider variable
Rename r10k::install::ruby to r10k::install::gem to match the $provider value
Move bundle installation to r10k::install::bundle
Add Gentoo Portage support

It uses the package from Gentoo Portage
Introduces dependency to gentoo/portage Puppet module
Add default value for $gentoo_keywords
Merge pull request #14 from tampakrap/gentoo_support

Refactor install.pp and add Gentoo Portage support