Add spec for be_enforcing, be_permissiv and be_disabled matchers (for SELinux mode check)
check_enforcing, check_permissive, check_disabled change marge to check_selinux
Merge pull request from #64 kenjiskywalker:support-selinux-matchers
Remove spec for get_stdout matcher because it is deprecated
Move check_selinux from commands/redhat.rb to commands/base.rb

Because selinux is available in Linux OSes other than redhat based OSes.
Pass 'selinux' to shared examples for selinux matchers