add db demo

demo for TopN and SetContent
rename url_fetcher to downloader
rename readabilitier to readabiliter
add summary_score
support atom file
add db_feed

db_feed provides basic db operations
add feed_parser

feed_parser Provide NewFeed, support atom and rss
add meta.go

meta.go defines basic feed structs
add opml.go

support parse opml
add rss/rss.go

move rss.go to rss/rss.go
add gfwlist

gfwlist will parse gfwlist.txt
add html_extractor

html_extractor will download a uri and extract its content
add html_fetcher

html_fetcher download a uri and convert it to utf-8 encoded
add image_crawler

image_crawler will extract a html's picture and download it
add image_fetcher

image_fetcher extract all feeds' image
add opml_parser

opml_parser parse a local opml file and save it to db
refine db.go

- remove comments
- remove all panics
- remove TerminateMongo
- remove DoInSession
- remove InsertChannel
- remove ImagesUnready
- remove EntryUpdateContent
- remove FetchEntryImagesExternals
- remove ImageUpdateState
- remove MarkReadEntry
add some files

- rename readabilitier to readabiliter
- rename url_fetcher to downloader
- move rss.go to rss/rss.go
- remove url_fetcher/url_fetcher.go
demo

- provide basic rss file parsing demo
- provide basic db demo
doc_fetcher is obsolete