fix(tests): Update to handle unverified users re-signing up.

* Use newest intern (1.5.0) so tests hang instead of timing out.
* Update all signUp tests to test both an unverified user re-signing up and verified users re-signing up with an incorrect password.
* Update fxa-client.js->signUp to take an options block that accepts `preVerified` and `customizeSync`

Closes #666
Merge pull request #670 from mozilla/issue-666-welcome-to-hell

fix(tests): Update to handle unverified users re-signing up.