Fix program exiting when asking for 2FA code.

* If the user has two-factor authentication turned on, we must
  only call the callback to `createHeaders` in the callback sent
  to `get2fa`, otherwise the program exits prematurely and the
  user isn't given a chance to enter a code.
* Title-case two success logs to match title-case of other logs.
* Remove "Creating token..." log as it's confusing when followed
  by "Existing token found!". However, this is personal preference
  and I can see how it would be useful. If the log is returned, I
  suggest adding a similar "Please wait..." log to the checking
  of 2FA, but I think the waits are so short that leaving both
  out results in a simpler UX.
Merge pull request #10 from rogerhutchings/get-token-fixes

Fix program exiting when asking for 2FA code.