Fix Boost.Sync tests breakage.

Re-add date/time type tags removed by commit 6636f49f13154555b3b9c9618fca5f03cf47a5b3. These tags are used by Boost.Sync to categorize different date/time types.
Added comments to the type tags.
Merge pull request #12 from Lastique/develop

Fix Boost.Sync tests breakage.