Support Dalli, optionally with Connection Pool

DalliProxy in #53 won't pick up an actual Dalli Connection Pool (my bad). This patch takes care of that.

On an unrelated note, [the code here][1] feels a bit messy already and seems to call for a cleanup like [this][2]. 

[1]: https://github.com/hakanensari/rack-attack/blob/92be2791ab7824c6eb5d984311c529d48f344f6c/lib/rack/attack/store_proxy.rb#L6-L30
[2]: http://solnic.eu/2012/04/11/get-rid-of-that-code-smell-control-couple.html