Enhancement and bug-fix release

This is an enhancement and bug-fix release, and all users are encouraged to upgrade.

Brief summary:

• Fix #242: Handle exceptions in the on_data callable.

This release has been signed with my code signing key:

Vinay Sajip (CODE SIGNING KEY) <vinay_sajip at yahoo.co.uk>
Fingerprint: CA74 9061 914E AC13 8E66 EADB 9147 B477 339A 9B86