Give L.CircleMarker.setLatLng a return statement.

Ever since e5bf57c4f78e4843c7268c2585104a88e52ac700, `L.CircleMarker.setLatLng` has not had a return statement. The API says it's supposed to return `this`. This branch resolves this issue.

This should be backpatched to v0.7 as a hotfix if possible. The issue does not exist in any release prior to v0.7.