Kill current buffer when opening previous item.

Otherwise buffers with read items hang around and distract user when he
tries to quit from item buffer to search buffer.
Ask url-retrieve to be silent.

This opens the doors for periodic automatic feed updating.
Make the intro header have clickable buttons.
Add elfeed-test-generate-date.
Add a test for date changes.
Re-sort entries when the date changes.

Before this fix, date changes would break the AVL tree.
Mask hooks on with-elfeed-test as well.
Test some more assurances about date updates.
Append the GC hook since it's non-essential.
Add n and p bindings to elfeed-search.
Cleanup feed titles.
Strip the namespace before determining feed type.
Expand feed type recognition to xmlns checking.
Add an xml-query test.
Add more syntax to xml-query (lists).

Lists are an OR matcher.
Add docstring to xml-query-strip-ns.
Support the bizarre RSS 1.0 format.

This is nuts: http://web.resource.org/rss/1.0/spec

Fixes Gmane feeds and #5.
Allow for nil titles and links in filtering.
Clean up some compiler warnings.
Allow elfeed-looks-like-url-p to accept non-strings.