feat(client): signUp accepts a `preVerifyToken` option.

`preVerifyToken` is an opaque token passed by trusted RPs to indicate a user
has already been verified and that FxA should skip verification for this user.

`preVerifyToken` is in addition to `preVerified` which is used by
unit/functional tests to create a user without going through the
verification flow.

fixes #126
tests(preverify): disable preverifytoken test unless using mocks
Merge pull request #127 from mozilla/issue-126-preverify-token

feat(client): signUp accepts a `preVerifyToken` option.