Add --relative command line argument for autoload structure testing

[ryanuber]

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.
This should not modify default behavior of the tool.

This closes #245

Example

puppet-foo/manifests/init.pp

class foo { }

Without --relative

$ puppet-lint --error-level=error puppet-foo
ERROR: foo not in autoload module layout on line 1

With --relative

$ puppet-lint --error-level=error --relative puppet-foo

This allows us to continue testing the general autoload layout if we purposely
compare relatively.