Configured request retry to avoid Github 500 Internal Server Errors

Improved the dashboard to retry failed requests and allow a single-threaded mode:

* Debug output has been enabled.
* Multi-threading can be now turned off by configuration.
* A backoff based retry has been implemented so failed requests are retried waiting a reasonable amount of time. Retries will wait exponentially until the configured max retries.

        ...
        GET https://api.github.com/repos/abiquo/flex-client-stub/pulls
        GET https://api.github.com/repos/abiquo/flex-client-stub/pulls/71
        GET https://api.github.com/repos/abiquo/flex-client-stub/issues/71/comments
        Request failed. Retrying in 1 seconds
        GET https://api.github.com/repos/abiquo/flex-client-stub/issues/71/comments
        Request failed. Retrying in 2 seconds
        GET https://api.github.com/repos/abiquo/flex-client-stub/issues/71/comments
        ...