refactor
add Quandl::Operation::Errors
add Quandl::Operation::Errors::UnknownDateFormat
Quandl::Operation::Parse.perform(data)
outputs dates as Date instead of Integer
invalid dates raise date_format_error with informative message
reject dates that preceed time
add qdate
Release 0.2.0. rename Parse methods to be more informative, add validations and informative errors for invalid data