Shorten code in rss_get_channel_data()
Store feed id to subscription list
Method to add subscription from feed.  Close #25
bug: mark read gone when overwrite crawled feed
Merge with previous revision of the same session
Move reduce() to compat
Merge branch 'bugs'
Coercing values into element objects  #26
Text can be automatically casted from string  #26
Mark can be automatically casted from bool

Fix #26
Merge nullable child elements
Properly merge subscription lists
Make crawler to immediately start when it’s called

Previously it starts when it firstly yields
Erase unneeded test

This test is in subscribe_test.py
Crawler return the feed as its entries sorted
Use concurrent.futures when it’s available
race condition
Coding style
Add atomic option for FileSystemRepository
Crawler append a <link rel="self"> to the feed when it's absent