Provide a constructor for a new transmission and client or make the zero value work better

[maplebed]

It's easy to leave off a config value when creating a client or transmission Sender struct by hand and the result will be undesirable. For example, forget to specify BatchTimeout and an idle libhoney will never send events until they reach the max batch size.

This can be fixed by being vigilant about always setting all the important members of the struct, or it can be fixed by using sane defaults for members that are not overridden.