Add XML metadata to output json/dictionaries

[bworrell]

Currently, some XML metadata is left out of dictionaries/json produced by to_dict() and to_json() calls. Namely, schemalocations and non-STIX namespaces.

Going round trip on a document (XML to python-stix to dictionary to python-stix to XML) may produce invalid XML instance data because of this lossy translation.

For example, non-STIX id namespaces need to be included in the output dictionary from to_dict() so that a document being built from that dictionary can include the appropriate namespace declarations.