Read state is now a stored document in CouchDB

We don't store the user unread status in the "user" object, but we
store a unique (userLogin, feedXmlUrl, articleGuid, readState)
document, called a "readstate" document.

This commit adds readstate document creation whenf etching new articles
from RSS feed.

NOTE: The implementation could be better, as we wait for all feeds and
all users to be fetched before creating these documents, this could be
improved.
Added unread count support for subscriptions list
Full support of the readstate object

Complete changelog:
- added a new API URL : GET /user/articles/:guid, to get the content of
a specific article without downloading the content of the other
articles in the feed
- updated readstate document format to contain more information, thus
reducing the number of requests needed
- added couch-wrapper function to update a the read property of a
readstate object
- removed an old non necessary CouchDB view
- updated frontend to take into account API changes
Unsubscription now deletes readstate objects

Fix #24

NOTE: We limit the number of concurrent DELETE request to 5 with
deferred.gate. CouchDB seems to fail when too many DELETE requests are
issued at the same time…
Merge branch 'refactor-couchdb-storage'