fix filtered markers being visible after spiderifying clusters

Currently all markers in a cluster are rendered by Spiderfy even if they were filtered.
Added check to see if _icon exists before acting on it
Merge pull request #22 from johnmcase/iconFix

Added check to see if _icon exists before acting on it
Merge pull request #21 from maxhoffmann/patch-1

fix filtered markers being visible after spiderifying clusters
Merge branch 'development'
Version 0.9.1