Added additional field attributes

Wanted to use this library in our apps tests for examining pdf field values.  The Field object was limited to a pre-defined set of fields.  I added additional common field attributes and support for handling non-pre-defined or unknown field attributes.  If a field attribute is unknown, a method is created for the ruby-ized field name and it's value set.  Names simply have the Field prefix removed and are downcased i.e. FieldFlags becomes flags.

Thanks for releasing this library.