Cleaned up Price\Value

Finalized Value properties.

#### Type
hash & persistence handling have been updated.

#### Converter
Now reads data_text, formatted as "<isVatIncluded>,<vatId>", and sets the appropriate value.

#### Value
Now has two properties: `(float)$price` and `(bool)$isVatIncluded`.
The Value constructor now accepts, for convenience:
- either an array of properties,
- or a price.