New features: amount and currency accessors

New features:

• $price->amount() and $price->exclusiveAmount() will return the exclusive money amount (including modifiers) as an integer string ;
• $price->inclusiveAmount() will return the inclusive money amount (including modifiers) as an integer string ;
• $price->baseAmount() will return the base (original, without modifiers) money amount as an integer string ;
• $price->currency() will return the underlying Money\Currency instance.

Bug fixs/Enhancements:

• Currency extraction from complex parsable currency strings has been optimized
• Currency recognition regexes have been fixed for some exotic currency unit symbols