Enable #parse to detect currency with signed amounts

  - parse can now detect the currency from strings with signs e.g:
    * -€9,99 => #<Money fractional:-999 currency:EUR>
    * €9,99- => #<Money fractional:-999 currency:EUR>
    * +£9.99 => #<Money fractional:999 currency:GBP>
    * £9.99+ => #<Money fractional:999 currency:GBP>