cpu_usage and cpu_shares should return nil if run command returns nil

NoMethodError - undefined method `strip' for nil:NilClass - is thrown
when calling Container#cpu_usage and Container#cpu_shares if
Container#run returns nil.
Use run(cmd).to_s.strip instead
Merge pull request #7 from nysa/master

Container#cpu_usage and Container#cpu_shares should return nil if Container#run command returns nil