Fixed `Provider.save()` to properly ignore stores which do not provide
a saveSync method. Also, fixed `save()` to properly save asynchronously
when an async `save()` method on a store is provided.
Removed the tests from `nconf-test.js` which expected `save()` to throw
or return an error when a store without `save()` methods was
encountered. Also removed a `console.log` from `provider-test.js`.
