Implement 'id' query tag

This is an "invisible" query tag, in that we don't actually use it as a
user available search term, but it will be useful for looking up
individual records.

Issue #251.
Add a basic download preview implementation

Based on the D2 mockup renders.

Issue #252.
models/search: Select all fields in searches

Issue #252.
views/download: Wrap results in preview-frame

Issue #252.
js/download: Import Stefan Goessner's json2xml

Issue #252.
js/download: Pettify XML output

Insert newlines and tab indentation.

Issue #252.
js/download: Implement XML format output

Issue #252.
js/download: Add 'available_at' attribute to data

Issue #252.
scripts: Add a run.sh script

The purpose of this is to reduce the latency between launching emacs and
getting on with some useful hacking.