Use dc:identifier as entry_id

Fixes #181
Merge branch 'patch-1' of https://github.com/dasevilla/feedzirra into dasevilla-patch-1

* 'patch-1' of https://github.com/dasevilla/feedzirra:
  Use dc:identifier as entry_id

Conflicts:
	lib/feedzirra/parser/rss_entry.rb