Update cache.go

Support Redis password
Merge pull request #6 from tuxmonteiro/master

Update cache.go - redis password support
Redis password param
Define default REDIS_PASSWORD (nil)
Update README.md
Merge pull request #7 from tuxmonteiro/master

Declared redis_password param and Readme updated
const initializer cannot be nil

const initializer cannot be nil. REDIS_PASSWORD adjusted.
Merge pull request #8 from tuxmonteiro/master

const initializer cannot be nil