support for identifying the resource type on a source description

The attached proposal adds a property to `SourceDescription` that can be used to identify the type of resource being described. The suggested list of resource types includes the following:

URI | description
----|------------
`http://gedcomx.org/Collection` | A collection of genealogical resources. A collection may contain physical artifacts (such as a collection of books in a library), records (such as the 1940 U.S. Census), or digital artifacts (such as an online genealogical application).
`http://gedcomx.org/PhysicalArtifact` | A physical artifact, such as a book.
`http://gedcomx.org/DigitalArtifact` | A digital artifact, such as a digital image of a birth certificate or other record.
`http://gedcomx.org/Record` | A historical record, such as a census record or a vital record.