Consistently return boolean
Make sure callbacks triggered respectively

Current version of Curb only trigger `on_failure' on http error 50x code.
Move 304 related code to `on_redirect', and use `on_complete' to handle
the rest.
Adapt feed spec

Conflicts:
	spec/feedzirra/feed_spec.rb
Fix specs to pull in #161

When we changed to doubles, I think we broke this PR. This fixes it and
makes it ready to be pulled in.
Write missing spec for add_url_to_multi

Specifies what happens when no compatible parser can be found.
Remove spec for recursion

If its hard to test, then you're doing it wrong, but I couldn't think of
a better approach, so I'm just removing this pending spec.
Add missing spec for parser exceptions

Specify what happens when the parser raises an exception.
Raise when parser invokes its failure callback

Inspired by a commit from @filiptepper on #159, I took a closer look at
stuff and wanted to get more specs around this behavior.
Extract callback for when the parser fails
Add PubSubHubbub hub urls as feed element

Conflicts:
	spec/feedzirra/parser/atom_feed_burner_spec.rb
	spec/feedzirra/parser/atom_spec.rb
	spec/feedzirra/parser/rss_feed_burner_spec.rb
	spec/feedzirra/parser/rss_spec.rb
add support for iTunes image in iTunes RSS item

add spec for iTunes RSS item image

add image to iTunes sample feed

Conflicts:
	spec/feedzirra/parser/itunes_rss_item_spec.rb
Bump version for 0.4.0
Update CHANGELOG for 0.4.0