Added delimiter attribute to PatternFieldGroup

Resolves #87 

**Implements the following proposal:** https://github.com/CybOXProject/schemas/wiki/Proposal:-Allow-User-defined-List-Delimiters

This pull request implements the following:
* Added `delimiter` attribute to `PatternFieldGroup`
* Set default value of the `delimiter` field to `##commaa` for backwards-compatibility
* Updated annotations on derivations of `BaseObjectPropertyType` which referred to the `##comma##` delimiter