Use "nerf-gun" to return "nerf darts" and higher level authify tests

@othiym23 @isaacs 

Supersedes both #73 and #74. Adapted the base functionality into the [nerf-gun] module which returns these "nerfed" URLs along with the appropriate bug fix. Kept the authify tests since they could be used for more of the edge cases that definitely need coverage in `authify` itself. 

[nerf-gun]: https://github.com/indexzero/nerf-gun