Crawler returns a Error when crawling failed

Now crawler does not stop even if crawling a feed failed.
add function 'get_all_feeds' to FeedList, FeedCategory
get_all_feeds returns `set`
Merge pull request #7 from Kjwon15/feedlist

FeedList.get_all_feeds() function
Check obj type using xml_url
Remove test print codes
Merge branch 'crawler' of https://github.com/viobo/libearth into viobo-crawler
If outline element has children(s) and not 'xmlUrl' that is a FeedCategory, not a Feed.