updateUser code simplification
Fixed a wrong redirection after signup
Rewrote couchdb-wrapper to store feeds and articles in database

also rewrote a propre init-db script
Re-implemented feed subscription

@Yimgo The code in `server.js` is a bit ugly, my apologies.
Re-implemented feed unsubscription
Implement feeds update.
Fetch articles from db instead of network.
Fixed empty server response when POST /feeds/update without any token
Rearchitectured database creation process

with better promise support
Replaced server-side map by a database map
Feed fetching store unread status
Implement #18.

It is buggy and does not handle properly concurrency.
@Paul_Mougel: it restores updateUser() function, of which were dependant the mark read/unread calls.
Improved feed update behavior

When updating the feeds, we concurrently:
- fetch all users;
- fetch all feeds, which allows us to fetch all stories, add them to
the database and infer from the database response code which article
are new.
When both promises are fulfilled, we update each user concurrently with
the the new unread articles. Thus, Each user object is now only updated
once, which fixes the multiple and concurrent edits done in
74fba1bdcc6cadd4aaa93fcc69cfd996fe25a14c
Merge branch 'refactor-read-status'