issue-18 expose backoff so it's configurable

Squashed commit of the following:

commit 24a9592c3290d4ae776b38d65e79323d9744e4f6
Author: Vanchi Koduvayur <vanchi@zendesk.com>
Date:   Fri Aug 23 16:13:56 2013 -0700

    expose backoff so it can be configured

    closes #18
v0.2.12