Improve Configuration API for defaults.

Configuration defaults in the API are already working fine, however there is no way to use them. This will make the following changes:
- Add some methods to `ConfigurationProvider` for loading configurations using specified default values and using `InputStream`s.
- Add a public constructor to `Configuration` so you can create empty configurations (with defaults).
- Changed `Configuration#get(String)´ to use the default values instead of null.