WebSearch: timestamp detection fix for empty sites

* Fixes get_database_last_updated_timestamp() so that it would not return
  None in case no records are present in the system.  This solves webcoll
  traceback issues when running on an empty site.

Reported-by: Ole Rønning <ole@tindtechnologies.com>
Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
redisutils: initial release

* Utilities to connect a Redis database.

Tested-by: Tibor Simko <tibor.simko@cern.ch>
WebSession: adds redis support

* Allows to store sessions in redis. (closes #1688)

* Introduces a new config variable CFG_WEBSESSION_STORAGE
  that can be set to mysql or redis.

Tested-by: Tibor Simko <tibor.simko@cern.ch>
BibRank: citation blobs in redis

* Stores citations dictionaries in redis for faster access when loading
  invenio.  The number of parallel queries used slow down the database
  at apache startup.  (closes #1689)

Tested-by: Tibor Simko <tibor.simko@cern.ch>
installation: redis and nydus in pip requirements

* Adds redis and nydus to pip requirements.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>