Made NumericProperty work with unicode strings.

Changed string type checks from "type(x) is str" to "isinstance(x, string_types)".
Also had to add str(x) to all parse_str() method calls.
Removed numeric_fmt str type declaration

Removed str method parameter type declarations.
Changed numeric_fmt type declaration from str to object.
Merge pull request #2141 from cayci/master

Made NumericProperty work with unicode strings.   References Issue #2078