Add default values support for form types

PR https://github.com/nelmio/NelmioApiDocBundle/pull/251 adds support for default values, but the formatter does not support this functionality, neither do any of the parsers parse out default values. 

This PR adds this support by adding default value support within the AbstractFormatter and parsing default values for forms. 

Other parsers, such as the JMSMetadataParser, should also add support to parse out default values and populate the default field.