MiscUtils: redis utils

* Utilities to connect a redis database
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.
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)
Travis Redis Test