add lib/serverspec/commands/linux.rb 

I've added lib/serverspec/commands/linux.rb 
and devided commands depending on Linux into it.

lib/serverspec/commands/{readhat,debian,gentoo}.rb inherits lib/serverspec/commands/linux.rb instead of lib/serverspec/commands/base.rb. 

* commands not depending on OS and distribusion should be defined on lib/serverspec/commands/base.rb
* commands depending on LInux (like SELinux and iptables) should be defined on lib/serverspec/commands/base.rb
* commands depending on Distribusion should be defined on lib/serverspec/commands/redhat.rb and so on.

The reason for this is here.
https://twitter.com/ftnk/status/328361052821667840