Allow scholar.py to query multiple pages

I noticed that the URL of Google Scholar's query URLs includes a `start` parameter, indicating the start index of the query.  By programmatically setting this parameter and looping until no more query results are returned, this modification will allow us to fetch lots more results than we could before.

I have noticed, however, that the tool quits after the first 1000 results, but it works great up until the 1000th item!  This limit does not appear to be a fault of scholar.py.  In a [sample search](http://scholar.google.com/scholar?start=980&q=%22ecosystem+services%22&hl=en&as_sdt=1,5&as_vis=1), Google Scholar will stop returning results after the 49th page (the 50th page is empty).

Thanks for this project!  It's been a huge help.