Revise content negotiation, result formats, and defaults

- Improve content negotiation to consider order of accepted content
- Remove `page` format, introduce `negotiate` format (new default)
- Update tests and sample queries on index page

This retains JSON as the default if any content is acceptable (*/*)
but will deliver HTML/RDFa if any content is acceptable, but e.g.
text/html is preferred (as typically requested by browsers).
Merge pull request #156 from fsteeg/result-formats

Revise content negotiation, result formats, and defaults