Implemented autoconvert to Date support for numbers and strings

Please consider this pull request. It implements autoconvert support for Dates whenever the input value is a string (parseable by Date.parse) or a number.

This change lets me expose collections using simple schema through a rest api, with seamless integration of dates attributes. It might be useful to other people, but probably you have a good reason why not implementing this autoconvert feature.

Thanks for your package!

All the best,
R.