Fix double-scaling bug

Turns out this package hasn't been working for a few months.

Add support for Jitter (#28)

* Expose Rate for better scaling control
* Expose other Retry values to allow for direct instantiation
* Use Phi as default scaling rate.

Add Reset() method

Clean up README

Preserve error when return from failed precheck

Resolves #23

Open Source

Merge pull request #7 from codercom/bugfix/jitter

Fix Jitter math.

Merge pull request #6 from codercom/refactor-proposal

More flexible refactor.