Fix spec failures

After changing the to_cfg method to be able to pass an alternate tab
size, the spec tests failed. This should now be fixed
Add note about puppetlabs_spec_helper in README.md

All the available spec tests depend on the puppetlabs_spec_helper gem.
This should be mentioned in the README because otherwise you will not be
able to actually run the tests.
Change map_fields method slightly

The map_fields method will now support a hash for optional options with
`section` beeing the only one allowed so far.