Rename libearth.feedlist to libearth.subscribe
Document might implement __bool__/__nonzero__
Subscription list become a pure document element

It's a necessary process to synchronize the
subscription list between multiple installations.
Stage subscription list
Merge branch 'subscribe'
SubscriptionList has to be mergeable to stage
Erase the unsed function
Fix update bug of subscription set
Filtered subsets
Merge branch 'master' of https://github.com/earthreader/libearth
Fix all Sphinx build errors
Remove useless print codes
Stage subscriptions test
Fix false positive reporting on Travis CI
Merge branch 'master' of https://github.com/earthreader/libearth
Change categories to dictionary to access Category instance
import reduce on python 3.x
Make iter(SubscriptionSet) to return unique set
write() can explicitly return chunk as bytes
Person is now hashable