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
    ...