Fix default CSP setting for Ace Editor

Like @sebastianrothbucher noticed in apache/couchdb-fauxton#5
the Ace editor needs base64 image data as image source for their
icons
Expose and add helper for tests

In order to make them reusable in the chhtpd integration-tests.
Additionally adding a post and delete helper.

COUCHDB-2462
Refactor couch_httpd_auth's AuthModule construct

We were missing the change necessary to upgrade user documents with the
move to pbkdf2 in a cluster. This also adds the ability for an
AuthModule to return a context that will be used when updating user
credentials.

COUCHDB-2491
Export session utility functions

Other auth solutions will want to reuse these functions. These should
probably be moved to a separate auth utility module but this seems like
the most prudent course of action.

COUCHDB-2491
Merge branch '2491-refactor-couch-httpd-auth'

Fixes #21
Return username on POST to /_session

When logging in with admin credentials and no user doc is
present, the name was `null`. Example:

`{"ok":true,"name":null,"roles":["_admin"]}`

closes COUCHDB-1356