Improved support for parsing dataset descriptions published as RDF

I've made a start at improving the support for parsing dataset descriptions published as RDF.

This covers:

* Generalising the RDFXML publishing format to cover additional RDF serializations, including turtle and n-triples. This Fixes #8
* The code now uses conneg to parse RDF. Fixes #7
* Dataset autodiscovery (use of rel=alternate) is now only used if there's no data available as RDFa, but there's HTML available from the discovery URL
* The RDF parsing code also now looks for VoiD datasets, so it should now work with the Ordnance Survey data
* I've added a dependency on fakeweb to allow us to mock out web requests for testing.

The above is covered by RSpec tests so hopefully I've not broken anything that the certificates app is reliant on. Would be useful to get some review of the code though.