AggregateUnitConverter changes

- Created separate AggregateUnitConversionProvider<TValue>;
- Created UnitConverter<TValue> class: derived from
AggregateUnitConversionProvider<TValue>, implements
IUnitConverter<TValue>, does conversion caching;
- Removed AggregateUnitConverter<TValue>;
- TODO: Add tests for UnitConverter<TValue>.
Fixed class name in readme file
Turned on xml doc generation on Release
Fixed unit tests for UnitConverter