Allow more than one titleType attribute per pbcoreTitle

[sroosa]

Reviewing the controlled vocabularies for titleType have left me realizing that there is more than one type of information that can be conveyed through this attribute. For example, many titleTypes refer to the scope of the title (Series, Program, Episode, Element), while other refer to the version or format (Working, Final, Original, Alternate).
We could make more specific attributes for Title so that each of these types gets its own attribute, but an easier fix would be to allow more than one titleType attribute per pbcoreTitle.