Addition of Google Custom Search Box to Docs

Solution to issue: shippable/pm#649

Image of search box:
![screen shot 2014-06-02 at 2 49 19 pm](https://cloud.githubusercontent.com/assets/3680260/3153934/c4d0940a-eaa1-11e3-8412-92fca0fac1a2.png)


Image of results after search:
![screen shot 2014-06-02 at 2 49 34 pm](https://cloud.githubusercontent.com/assets/3680260/3153935/cb97062a-eaa1-11e3-942a-cfca7b0cd369.png)