blank strings validation

1 of 2 pull requests - one here and the other on meteor-collection2.

What these pull requests are for, is so that if you want to submit a field with a blank string and not have that field set to "optional," then what the validation will do is first clear out the field and then fail.

This update passes through the blankStringsAllowed option all the way through to the validation check so that in this situation, you will not get a validation failed for a blank string set to a required field.

You may want to manually merge since the patch is probably not implemented in the best way and also the atmosphere package files/settings have been changed a little so that the fix could be used before it was implemented here.