Issue 307 basicauth revisited

Oh Kay!  Not only does this eliminate our backbone.basicauth.js dependency, but I believe the code is actually less complicated, more clear. And it inherently integrates our basic auth provisions with any alternative ajax handlers that we plug in to spiderOakApp.ajax (using the alternativeAjax configuration setting).

(I've also cleared some elusive, untidy testing state that was leading to obscure failures.)

Fixes #307