Create XSD Schema for FIX dictionary files

Fix xml dictionary documents (FIX44.xml etc) should have a defined schema (XSD).
The schema can be used for...
authoring customer fix data dictionaries.
(ii) validating documents at both build and runtime.
(iii) configuring the DataDictionary (via JAXB).

At the moment the DataDictionary.load() method pulls in an org.w3c.dom.Document instance and then iterates through it using hardwired node names doing some adhoc validation along the way. It would be much better to load a validated document as a type safe object and iterate through that.

The schema should fit the existing doc layout with as few changes as possible (probably a namespace declaration will be required).