Natural language parser returns date that parse_edtf does not accept

[blms]

The following raises an EDTFParseException:

from edtf import text_to_edtf, parse_edtf
parsed_natural = text_to_edtf('circa 17th century')  # returns "16xx~"
parse_edtf(parsed_natural)  # raises EDTFParseException

[aweakley]

This is resolved in #56