Added Module and test for if a string is a valid currency

In lib/Data/Validate/Currency.pm
  Added POD and is_currency method. Returns 1 if it is a valid currency,
  else returns 0.

In t/test_strings.t
  Added subtests:
    One dollar
    One hundred dollars no dollar sign
    One thousand dollars
    Bad data
    One million dollars
    999 million dollars
    One billion dollars
    One trillion dollars
    Quadrillion dollars
    Quintillion dollars
    Sextillion dollars
    Octillion dollars