Hide disclaimer using sessionStorage

Hides preview disclaimer after it's been seen once. Helpful in situations such as if the user is browsing the site on a phone. Currently the message fills a large person of the viewport, but this version only shows up once per session.

![Screenshot from Moto X (2nd Gen)](https://cloud.githubusercontent.com/assets/6354860/4672641/27d70442-5593-11e4-8fa4-885d23752b83.png "Screenshot from Moto X (2nd Gen)")


 Using `sessionStorage` hides the message for the rest of their visit, but when the user comes back the sessionStorage is made blank by the user, so that the message is shown yet again. If prefered, references to `sessionStorage` can be replaced with `localStorage` so that the message is shown once and hidden permanently. You can check [here](http://caniuse.com/#feat=namevalue-storage) for compatability details. For details, see comment in main.js.

The first few commits also modify the README, but final commit takes a clean branch pulled from the main repo and adds in my changes.

The JS has already been minimized, and it also gets rid of the erroneous
```javascript
$('.add-tooltip').tooltip();
$('.add-popover').popover();
```
that was missed when moving code to `$(document).ready()`.