Add base commands helper (for test use mainly)
Separate build command method for testability and add custom path in this method
Add spec for build_command method
Add rake task for specs of heplers and backend
Remove adding default PATH

Error ``sorry, you are not allowed to set the following environment variables`` shows up with sudo secure_path setting.

So I've removed adding default PATH.
Merge pull request #133 from mizzy/add-custom-path

Add custom path to the command