Add package goback

Adds exponential backoff package.

README.md

@@ -578,6 +578,7 @@ Join us on IRC at **#awesome-go** on freenode [web access](http://webchat.freeno
 * [coop](https://github.com/rakyll/coop) - Cheat sheet for some of the common concurrent flows in Go.
 * [delve](https://github.com/derekparker/delve) - Go debugger.
 * [fastlz](https://github.com/fromYukki/fastlz) - Wrap over [FastLz](http://fastlz.org/) (free, open-source, portable real-time compression library) for GoLang.
+* [goback](https://github.com/carlescere/goback) - Go simple exponential backoff package.
 * [godropbox](https://github.com/dropbox/godropbox) - Common libraries for writing Go services/applications from Dropbox.
 * [gopencils](https://github.com/bndr/gopencils) - Small and simple package to easily consume REST APIs.
 * [goplaceholder](https://github.com/michiwend/goplaceholder) - a small golang lib to generate placeholder images