bump to version 2.60.17
Merge remote-tracking branch 'origin/feature/automatic_sessions' into devel

Conflicts:
	globaleaks/__init__.py
	globaleaks/db/base_updater.py
	globaleaks/db/sqlite.sql
	globaleaks/db/update_13_14.py
	globaleaks/tests/handlers/test_node.py
removed old not useful exceptions
fixed various bugs inserted in https://github.com/globaleaks/GLBackend/commit/a084b384055c7f8c305d2a8c80de757c770bd1f1
minor fix against issue https://s3.amazonaws.com/archive.travis-ci.org/jobs/36345624/log.txt

i've not clearly understood why this has happened by the way this condition may happens also in case of race conditions between explicit doce flow deletion of the object (let say logout on sessions) and the expire time.
prepared tests skeleton for OpenPGP-python integration
implemented Cache class for read only API
implemented update of cached API answers
adoption of Cache for public API in /node
update of Cache after wizard is complete