changes: form field from DecimalField to MultiValueField

I did a major rewrite of the entire from section. I switched from a decimal field subclass to a 'proper' MultiValueField.
That has certain advantages and reduces the codebase. That way we can make use the latest changes in Decimal and SelectFields such as l10n and other stuff.

I updated and added some tests as well, travis-ci say's it's all well.
BTW: Hi from Jake, he told me you're the new maintainer.