PORT-111 Aggregate type encoding helpers

- Renamed TypeImplementation to BasicTypeImpl to save confusion with the new
  AggregateTypeImpl class
- Encoding helper implementations written for all aggregate types except for
  HLAvariantRecord
PORT-111 Added some extra commenting in HLAopaqueData
Merge pull request #19 from michaelrfraser/port111-encodingHelpers

PORT-111 Aggregate type encoding helpers