Support setting the sender

[dehnert]

zsend broke with the Scripts update, and I didn't feel like updating it, so I switched zcommit to use plain zwrite instead. zwrite doesn't support setting the zsender, so we lost that ability. Evan Broder's python-zephyr library does support this, but it always authenticates to the zephyr servers, so you can't use it without a keytab.

We could switch to python-zephyr and fix this in one of two ways:

• get a keytab
• stop requiring tickets to use python-zephyr (ie, fix Support sending without authenticating dehnert/python-zephyr#1)