- fixes styles written as invalid properties
- fixes part of #129 (issues 2 an 3), added test images

- added possibility for class-specific default attribute values
- fixes load/save of W3C example pservers-grad-06-b
- fixes #397

- suppress warning CS1591: Missing XML comment for publicly visible type or member
  to avoid to overflow the build log with warnings
- fixed the rest of the documentation warnings in SVG project
- ignore all failing tests for the time being

Added parameter to the SvgAttribute indicating whether the storage is
the _attribute dictionary.  If true, the output will only be serialized
only if the key is in the dictionary.  This structure is an alternate to
the SvgDefinitionDefaults class.