Add simple :pan control to the sample players.  

I wanted to pan some samples and found I couldn't.  So, I thought I'd try a pull request to add the feature.

Example usage:

;; mono sample
(def kick (sample (freesound-path 2086)))
(kick :pan -1)

and

;; stereo sample
(def sol-do (sample (freesound-path 44929)))
(sol-do :pan 0.5)