Add authentication to mailinabox-daemon; resolves #67

I've tried to create the simplest authentication scheme possible, so it can be thrown away in a month or in a year or whenever the fully-featured authentication + authorization scheme is completed.

Adding authentication to the service itself ended up being pretty clean. The biggest hassle was updating all the client code (and examples) to pass an authentication token. I realize it's going to be a bit of a pain to deal with going forward, but I think it's a small price to pay for such a huge security win.

Let me know if I can change anything to make it easier for you to accept and merge.