Implemented basic Price FieldType.

> Part of [EZP-23034](https://jira.ez.no/browse/EZP-23034)

Basic implementation, based on Float. No external storage yet. SPI & Unit tests are there.

Manual VAT handling is being worked on in #14
not like https://github.com/ezsystems/DemoBundle/blob/master/DependencyInjection/eZDemoExtension.php#L20 then?