Fixed bug where you couldn't make visible a group of markers (overlay) created as not visible by default. Added example on how to create groups of markers and how to make them visible / not visible on demand.
Merge pull request #225 from yagoferrer/master

fix(build): Fixed bug where you couldn't make visible a group of markers (overlay) created as not visible by default. Added example on how to create groups of markers and how to make them visible / not visible on demand.