Bug 1051806: Add /fxa-register endpoint.

@Sancus r?

This may conflict a bit, or at least could benefit from, the work
you're doing to move more ET stuff to tasks, but we can always clean
up after all the merging.
Someday it might be nice to have a small class for these values that aliases the nicer names as properties to the ExactTarget names, as I see a fair amount of other cases where you convert between the two.

Obviously not for this PR, just an idea.