Add a `failureHandler` parameter to `#gate()`.

`#gate()` now accepts an optional callback function that is invoked
with the signature `(request, response)` if a feature is missing. The
`httpcode` option will be ignored if a callback function is specified.
Bump version.
Merge pull request #2 from kitcambridge/failure-handler

Add an optional `failureHandler` parameter to `Reflip#gate`