initial commit on savepoint support for mail messages
- trying to better handle multiple levels of nested transactions
- removing the resetTransaction function
cleaned up docs, added some transaction checking to join_transaction
Get tests passing on all supported versions.
Housekeeping.

Import cleanups.

Drop excess whitespace (vertical and trailing).

Inline 'is_resource_in_transaction' for clarity.

Conform to repoze style-guide.

Etc.
Coverage for MailDataManager.join_transaction.
Cleanups;  Avoid munging attr on error in MDM.join_transaction.
Explicit coverage for MDM.{commit,abort}.

Replace their asserts with ValueErrors.
Remove fossils:  abort_sub, commit_sub.
Full coverage for MDM's TPC methods.
Make 'transaction_manager' a ctor argument for delivery managers.

Also, convert non-test asserts to ValueErrors.
Explicit coverage for AbstractDeliveryManager.send.
Rip out debug logging.
Merge pull request #28 from repoze/pr24-savepoints

Pr24 savepoints