Add Serverspec::Helper::OSDetect
backend can take specific commands object

backend.check_os cause infinite loop with Serverspec::Helper::OSDetect
Add check_os method
Rename OSDetect to DetectOS
Modify spec_helper to use DetectOS helper
Fix README
Remove unneeded return.
Fix README
Merge pull request #56 from mizzy/auto-os-detection

[WIP] Automcatically os detection