Cope with unhandled rejected promises in /token handler

[nulltoken]

Fix #141

PR Checklist

• I have run npm test locally and all tests are passing.
• I have added/updated tests for any new behavior.
• If this is a significant change, an issue has already been created where the problem / solution was discussed: [N/A, or add link to issue here]

PR Description

Express 4.x doesn't properly cope with rejected promises by default.
Delegating to next() invokes the standard error handler.

[nulltoken]

@poveden Not ready yet. I still have to write a unit test to prove the code does what it's supposed to do.

[nulltoken]

@poveden Ready for review. Sorry for the diff noise. It looks like a big chunk of change, but it was really a matter of wrapping the token handler in a try/catch.

I've also taken the opportunity to create #143