Equal height utils function for DE pricing grid vertical alignment

This PR addresses MKT-828, https://github.com/optimizely/marketing-website/pull/726, https://github.com/optimizely/marketing-website/pull/738.

Testing instructions are below.  Note that the responsive version of this function does not work for the Pricing page because our grid elements we are targeting are in columns and not rows.
Writing my steps so we're on the same page:
1) Copy paste code from [this page](https://github.com/optimizely/marketing-website/blob/dfoxpowell/pricing-cta-vertical-align/website-guts/assets/js/utils/equal_height_grid.js) into the console.
2) `$('.plan-box-cont').css('display: flex')`
3) `$('.plan-box-cont').css('flex-direction: column;')`
4) `$('.plan-box-content').css('flex: 1;')`
5) ```window.optly.mrkt.utils.equalHeightGrid({
  selector: '.plan-box .cta'
});```

When I do that, the pricing boxes look like this:

![screen shot 2014-12-03 at 1 21 14 pm](https://cloud.githubusercontent.com/assets/2864449/5286116/88585920-7aef-11e4-805a-e199684034a3.png)

If I change the selector to `'.plan-box-content'`, I see this:
![screen shot 2014-12-03 at 1 25 35 pm](https://cloud.githubusercontent.com/assets/2864449/5286140/c1075140-7aef-11e4-934e-20652ae4114f.png)

We want the boxes to look like the second image as specified in [MKT-828](https://optimizely.atlassian.net/browse/MKT-828)

Let me know if I messed up anything!