Add to_json method

[mociepka]

It would be nice to have to_json method or to_dict or json serializer in prices. Now I use:

price = {name: str(value) for name, value in obj._asdict().items()}
del price['history']

but this doesn't look good.

[patrys]

I'd much rather finish implementing my idea that turns a Price into two Amounts.

[pcompassion]

+1 could serialize price during checkout process for instance and use it.