Use boolean values for 'unique' attribute

As defined in: https://github.com/doctrine/doctrine2/blob/master/doctrine-mapping.xsd#L294

Same as 'nullable' attribute. 

It was being exported as a "1" for TRUE and "0" for false
Added test to ensure boolean metadata is properly exported/serialized to XML
Merge pull request #984 from holtkamp/patch-1

Use boolean values for 'unique' attribute