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
I've not seen this. The two includes should be included automatically (because they're in the shared header at https://github.com/cloudant/CloudantQueryObjc/blob/master/Example/Tests/Tests-Prefix.pch)