Rethink fetcher limit

[koppor]

Currently, the fetcher seems to fetch 50 entries only. We changed that back a while ago.

This is not good for a good literature survey. This cannot be conducted using JabRef with this limit.

When we focus on having JabRef as tool for surveys, we should offer querying for authors, title, exclude strings, ... -- maybe even with a auto-generation for differnt fetchers. This would make surveys much easier.

[tobiasdiez]

Most fetchers already contain the option to specify the start index of the search result, so it should be relatively straightforward to implement paging.