Fixes a bug causing emails to be sent with the same timestamp when re…

…using a

`MailYak` instance (#50).

Thanks to @jacksgt for both identifying, and fixing the issue!

Chunked splitter writes (#34)

* splitter: correctly handle chunked writes

Previously a chunked write that did not neatly end the encoded line was
handled incorrectly, resulting in overly long line on the next call to
Write().

The lineSplitter has been changed keep a running count of bytes wrote,
allowing chunked writes to be split into lines of the configured length.

Fixes #32.

* Refinement of new splitter (#33)

* Add test for longer binary file

* Change linesize only after taking partial chunk

* Typo fixes

* Replace arbitrary use of constant.