[patch] minor refactor by reusing 'New' and added more tests

[major] implemented fmt.Stringer interface, so currency.String() now …

…does not accept any parameters

[-] introduced 2 new configurations, 'PrefixSymbol' and 'SuffixSymbol' which will respectively prefix or suffix the currency symbol to the string

[minor] implemented fmt.Formatter interface

- 's' verb would produce the full currency string along with its symbol. equivalent to c.String(true)
- 'd' verb would produce the main integer part of the currency, without the symbol
- 'm' verb would produce the fractional integer part of the currency, without the symbol
- 'f' verb would produce the full currency string without its symbol. equivalent to c.String(false)
- 'y' verb would produce the currency symbol alone

stable release