Node harmony Map doesn't support forEach

[villadora]

which make it hard to remove the dependency to 'harmony-collections' in the future. No iterator make the harmony implemented Map useless, because we don't know which is stored in it.

And now once people open node '--harmony' option, will make the native Map, the one without 'forEach' implementation, just get up, that will break this lib.

Make an option to let user disable the Map usage in this lib.

Without Map, all the keys will be treated as string.