Serialization Bug Fixes

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.
Serialization Fixes

More serialization bug fixes and better fallback for font definition
problems.
Merge pull request #105 from erdomke/master

Serialization Bug Fixes