redisutils: initial release

* Utilities to connect a Redis database.

Tested-by: Tibor Simko <tibor.simko@cern.ch>
WebSession: new Redis session storage backend

* 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 pre-requisites

* Amends the installation guide WRT Redis.

* Adds Redis and Nydus to pip requirements.

* NOTE Invenio now uses Redis for session management by default.
  Please install it or change CFG_WEBSESSION_STORAGE settings.
  See INSTALL file for more details.

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