Failure backoff

Implements #30 

This is based heavily on the raven-python code, with some tweaks to make it more ruby-ish.

Let me know if any changes are needed. I'm not sure if you use a specific ruby styleguide.
Please remove the parentheses from any [method calls with no arguments](https://github.com/bbatsov/ruby-style-guide/blob/master/README.md#no-args-no-parens).