Fix: not getting entire feed on url change

If the first entry of a feed changes its url feed.new_entries will
then contain all entries of the feed, even old, already seen ones.

find_new_entries_for only looked for the url of the first entry from the
last run to determine when the list of already seen entries starts. Not
finding the url, it did add all entries to new_entries.

Instead of using url, using id (which has a fallback to url) fixes this
bug.
Actually fix #70 - update should return hash for multiple feeds, else feed object
Added the :ssl_verify_peer flag to the easy_setup method
Use Time.parse_safely in Feed.last_modified_from_header.

This helps prevent parse failures when there is a problem with the last
modified header. For example if the Last-Modified header is empty like on
http://feeds.kottke.org/main.
Allow whitespace between attribute name, equals sign, and attribute value when checking for Atom or iTunes formats
added spec to pull atom entry urls even when they don't conform to the standard.