Atom feed on search results

Opening for discussion, not quite ready for merge yet,

Fixes #561 

I've changed the URI for the dataset feeds, and added feeds for `/datasets` and the search. Only issue is I want to add pagination to the feeds as per http://tools.ietf.org/html/rfc5005, but it seems that the RSS library that's included in stdlib doesn't support pagination (at least as far as I can see). 

I could throw away the RSS generating part of the code and just put them in the builder view, which would give me more flexibility, but wanted to see if anyone (particularly @JeniT) had any views first.