Only clear the markers cache for markers created by `features()`

This fixes the problem that tooltips are themselves just markers but don’t share
the same `data` and are not added to the marker cache at all.
See https://github.com/mapbox/markers.js/pull/52#issuecomment-14909544
Merge pull request #53 from Swatinem/gh-pages

Only clear the markers cache for markers created by `features()`