fixed bug where required is not respected for fields. #13

not sure when this broke. passing required through to validator.

don't try to JSON.parse() undefined or empty string value, instead just
call the validation method passing in the required param.

added required property/list to json-forms -> json-schema translator.

changed convertToMuvuku() to use empty string instead of 'undefined' in
the message.

probably still needs some work, mainly in translating the muvuku
validations like length:[1,5] to json-schema like minlength,maxlength.
0.1.0-beta.12