Fix conflict between function and class name in Swift

Declaring the delegate method the same name as the class name leads to a conflit between function and class name in Swift.

There is also the discussion about the same issue here : https://devforums.apple.com/message/1002447#1002447.
Merge pull request #513 from couchbase/feature/issue_512

API: Renamed -[CBLDocumentModel CBLDocument:didChange:]