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.
Merge pull request #131 from bhaak/fix_new_entries_by_using_id

Fix: not getting entire feed on url change
Remove cultural reference from description [#135]
No need to set date in the gemspec.

The default is to set the current time / date. The version of rubygems
that is currently shipping with 2.0 no longer requires date before
loading the gemspec so `Date.today` isn't available.
Update attribute should return true / false if it could update.

It is theoretically possible that the new value could be nil for all
options meaning that `@updated` would be false / nil even after a
successful run.
Merge pull request #136 from collectiveidea/ruby-2.0.0

Compatibility fixes for Ruby 2.0 (rc2)
Add @gaffneyc's PR to changelog
Merge remote-tracking branch 'original/master'

Conflicts:
	HISTORY.md