Support connecting using a connection string

This adds support for using a connection string like the ones you get on heroku. I've verified that it works on heroku (rediscloud).

Used binary for the string as that seems to be the new way strings are handled in erlang? If so, maybe it's time to change the signatures of start and start_link?

Also reordered the metods in call-order and removed some duplication.

I'm not very experienced with elixir or erlang, so please tell me if anything can be improved.