A small fix making HOST key non-mandatory

Due to mandatory HOST key it was impossible to create connection_pool.
It is possible now. See the example below:
pool0 = redis.ConnectionPool(host=...,port=...,db=0)
flaskApp.config['REDIS0_CONNECTION_POOL'] = pool0
r = Redis(flaskApp, 'REDIS0')
Merge pull request #8 from rolikoff/master

A small fix making HOST key non-mandatory