sanitize_author: more complete regex, ref #22
test_author_abuse_quotes: provide a possible resolution
sanitize_author: ensure that author name is stripped of whitespace
sanitize_author: handle empty 'name' according to issue #22 discussion

Following the fast-import spec, author fields containing only an email
address now render as

    <the@email.address>

instead of

    unknown <the@email.address>
Merge pull request #38 from jedbrown/sanitize-author

Sanitize author, closes #22