Added optional provider defaults to base_backage and extension classes

Not having `undef` as a default provider was causing errors similar to:

```
err: Could not retrieve catalog from remote server: Error 400 on SERVER:
Must pass provider to Php::Contrib::Base_package[cli]
```

Making the extension and php::contrib::base_package defines use `undef`
as a default still allows provider selection while falling back to sane
puppet defaults.
Merge pull request #28 from dbeckham/optional_defaults

Added optional provider defaults to base_backage and extension classes