Re: [webpayments] Expression of monetary amounts (#40)

The use of `mantissa` is likely to confuse some people. Sometimes it is used interchangeably with `significand` by computer scientists when talking about floating point numbers, but with respect to mathematics, it is understood to be the [fractional component](http://mathworld.wolfram.com/Mantissa.html) of a number:

```
For example, for x=3.14159, the mantissa is 0.14159.
```

Furthermore, using `significand` does not remove the possibility that people will try to use decimal points in its value. I realize the IDL has specified that it is a `long long`, however, as it is perfectly valid to use a decimal point in a significand; I can see people trying to use one. As one other point, perhaps we're not concerned about values this large (or small), but we could start seeing problems with JSON parsers if we're using numbers and not strings for precise values.

In my view, it would be better to just use a string that represents an arbitrary precision number that may contain a decimal point.

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