Move limting and skiping of results into class method

To speed up testing, the skiping and limiting of results
has been moved into a class method. Enabling testing of
the limiting and skipping code without excuting a full
query
One of the benefits of extracting a separate method is that it allows the checking to be improved while keeping the tests concise:

```objc
NSArray *results = @[@"doc1", @"doc2", @"doc3"];
NSArray *trimmed = [CDTQQueryExecutor applySkip:1 andLimit:2000 toResultSet:results];
NSArray *expected = [results subArrayWithRange:NSMakeRange(1,2)];
expect(trimmed).to.equal(expected);
```