Wrap &comma; entities with CDATA

This is a fix for issue #28 in CybOX 2.0, until a language-supported feature can be added in CybOX 2.1.

By wrapping any `&comma;` entities in a CDATA block, the parser won't try to resolve the entity, and will pass them unmodified to the python-cybox library, where it can be property identified as a literal comma as opposed to a list delimiter.

Once CybOX 2.1 is released (perhaps with a different escape sequence for literal commas), we will have to handle the compatibility between them (depending on `cybox_major_version` and `cybox_minor_version`). This is true regardless, for any other changes that occur in 2.1, so python-cybox 2.1.X will need a compatibility layer. 

For now, this allows literal commas to be used in CybOX content, which was previously impossible.

Closes #28.