Don't rerun query every time you want a new page; just call .range()

As discussed in #4, this is a patch to stop rerunning the query every time you advance to a new page. It depends on ibm-js/delite#174, which I just checked in.

It's mainly for efficiency. If your query involves a sort then you want to avoid resorting on each new page. And even if you aren't sorting, the query operation for user defined stores may be expensive, depending on the implementation.

Replaces #5.