Fixing source addition to objects created from STIX.

When the objects are created using their `from_*()` function for cybox
or stix they will no longer come with the source being added. The STIX
Parser now handles adding the source after the object has been returned.

Also fixed some copy/paste issues with docstrings.
Merge pull request #212 from crits/stix_sources

Fixing source addition to objects created from STIX.