Support for ranged fetch specifications

This patch adds support for fetch specifications with a range to H2 and PostgreSQL plugins taken from the MySQL plugin.

Example:
```java
EOEditingContext ec = ERXEC.newEditingContext();
ERXFetchSpecification<City> fs = new ERXFetchSpecification<City>(City.ENTITY_NAME);
fs.setSortOrderings(City.SIZE.ascs());
fs.setFetchRange(new NSRange(10, 5));
NSArray<City> cities = ec.objectsWithFetchSpecification(fs);
```

This will fetch rows 10 through 15 from table City.

If by mistake you have set a range _and_ a limit (```fs.setFetchLimit()```) the range will win and the limit of your fetch specification will be changed to 0.