Add CDATA wrappers to delimit content that contains commas

The XML parsers won't try to resolve the &comma; entities
Add test for list of strings containing commas
Wrap entire element in CDATA rather than each item in list.

Previously, if a property was a list with several items containing commas, each would
be wrapped in its own CDATA block. Now, the check is performed on the entire string
after serializing a list to a comma-separated string.
Merge pull request #40 from CybOXProject/cdata_commas

Wrap &comma; entities with CDATA