Update jquery.peity.js

Performance fix via length caching for a loop.
Sorry, I must have misunderstood you. I meant that the reason I declare `var i` outside of the for loop is because it's used further down on [line 160](https://github.com/benpickles/peity/blob/1e290b5a6ea2f92eb8fe4dc10d8e2e10ea67ef3a/jquery.peity.js#L160) (along with `length`) and I find it easier to grok what vars are around.

Could you get rid of this `i = 0` and re-add the `var i = 0` above please. (By the way this change also makes `i` a global)