Add build status

Author: shawncplus

README.md

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+memoization-behavior [![Build Status](https://travis-ci.org/shawncplus/memoization-behavior.svg?branch=master)](https://travis-ci.org/shawncplus/memoization-behavior)
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 `Biddle.MemoizationBehavior` allows you to define a computed property
 as memoizable (http://en.wikipedia.org/wiki/Memoization) with `_memoizeComputed`
 or more generally memoize any given function with `_memoize`