Cleaned up Price\Value

Now has two properties: (float)$price and (bool)$isVatIncluded.

hash & persistence handling have been updated.
The Converter now reads data_text, formatted as "<isVatIncluded>,<vatId>",
and sets the appropriate value.

The Value constructor now accepts, for convenience:
- either an array of properties,
- or a price.
Merge pull request #18 from ezsystems/value_cleanup

Cleaned up Price\Value