money: implement combined operations

Merge pull request #11 from govalues/arbitrary-scale

money: allow scales smaller than currency scale

money: improve table formatting

money: improve documentation and test coverage

money: implement new constructors and methods

money: implement sql driver methods and enable linters

Merge pull request #6 from govalues/quo-rem-method

money: implement Amount.QuoRem method

Merge pull request #5 from govalues/int64-scale-argument

amount: Implement scale argument in Int64 method

Merge pull request #4 from govalues/error-management

money: implement panic free methods

Merge pull request #3 from govalues/improved-documentation

money: improved documentation