Revised Resque_Redis constructor to accept passwd

Also,
* moved setting of default server into Redis_Resque
* removed option to format server as "redis://"
Added ability to send password to worker script

As part of this, also:
* added mention of backend env vars to README file
* moved handling of default database to Resque_Redis
Removed unused condition
Added back parsing of "redis://" server format
Revised commit messages