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.
test case for `crawler.py`
Raise CrawlError when something goes wrong.
Logo artwork  [ci skip]
Logo artwork on readme  [ci skip]
Raise the logo art on readme
Remove a radiant line
Use opener instead of httpretty module
Merge pull request #11 from viobo/crawler

Use opener instead of httpretty module
Erase httpretty in dependencies
Merge pull request #12 from viobo/crawler

Erase httpretty in dependencies #11 #10
Merge pull request #9 from Kjwon15/master
Refactoring on urllib2 mocking