Support Fitbit API change about HTTPTooManyRequests

This PR adds support for HTTPTooManyRequests error, that Fitbit will start using from May, 27th to signal the client that she exceeded her rate limit.  This improves compliance with RFC6585 (http://tools.ietf.org/html/rfc6585#section-4).

A HTTPTooManyRequests exception is added, and the Retry-After header content, which specifies how many seconds the client should wait before the next requests, is added to the exception as the `retry_after_secs` attribute.

This PR can be merged before May, 27th, because it's an incremental modification that doesn't break support of existing API.