Added 404 header support

More efficient error support
Refactored 404 implementation
Merge remote-tracking branch 'upstream/master' into 404
Fixed tests
Enhanced callback support
Merge remote-tracking branch 'origin/master' into 404
Refactored way to check wordpress 404 status code
Merge pull request #32 from Takeatea/404

Added 404 header support
Merge remote-tracking branch 'upstream/master'