Move backends to a separate directory
Move OS helpers to a separate directory
Rename backend modules as Serverspec::Backend::*
Rename helper modules as Serverspec::Helper::*
Use classes for backends

  Use an helper for backends again
  Instantiate backends with commands
  Remove specific implementations from matchers
Merge remote-tracking branch 'pr/36/head'
Add obsoleted helpers to show warning message if obsoleted herlpers are included.