Added --relative command line argument.

This argument will cause the autoload layout class plugin to ignore the
top-most directory in the structure. This is useful for the common case where
the folder name is not the same as the module name in raw source code.

Example:
puppet-foo/manifests/init.pp has 'class foo { }'
puppet-lint puppet-foo returns errors
puppet-lint --relative puppet-foo returns no errors

This allows us to continue testing the general autoload layout if we purposely
compare relatively.
Modified --relative CLI description to be more understandable
Modified --relative description to indicate that it will compare relative to the module root - not the working directory.
Merge pull request #252 from ryanuber/relative_option

Add --relative command line argument for autoload structure testing