Add subject filtering mechanism
Specify Serverspec::Commands::Base explicitly

Because in spec for serverspec itself, commands should be included for each OS.

But in Serverspec::Filter, commands are not defined because it's not included.
Call subject filter before examples run
Add spec for linux kernel parameter checking
Proc of Ruby 1.8 does not have source_location method
Fix the position of empty line
Merge pull request #69 from mizzy/check-kernel-params

Support linux kernel parameter checking