Avoid import of MUT at module scope.
Raise ValueError (rather than assert) if message passed to 'send()' invalid.
Force conversion of message to bytes.
Add a transaction_manager attribute to the mail delivery utility.

This allows one to easily override the transaction manager
used when sending mail.
update changelog and contributors
Adjust changelog.
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
Changelog for PR #28.
Typo
Add URL for online docs.