First attempt at adding .Fuzziness to suggest

- Changed the _Fuzzy property in CompletionSuggestDescriptor to
IFuzzySuggestDescriptor
- Updated FuzzySuggestDescriptor to implement IFuzzySuggestDescriptor
- Added FuzzinessSuggestDescriptor implementing IFuzzySuggestDescriptor
- Added .Fuzziness method to CompletionSuggestDescriptor
- Added unit tests in CompletionSuggestTests

Issues:
- Both the Fuzzy and Fuzziness methods returns a
CompletionSuggestDescriptor<T>, so that means you can chain .Fuzzy and
.Fuzziness. If you chain them, only the last one applied will be used.
That might not be a problem, depending on what level of knowledge is
expected from the developers using NEST.