Add styling for cards (first draft)

This is my implementation of [cards](http://www.google.com/design/spec/components/cards.html), inspired by the work of [Kevin Gimbel](http://codepen.io/kevingimbel/pen/GCDep).

In this first version, I only have styling for square cards. Later we can add other types of cards (rectangle cards, cards with solid color as background,...).

A card will expand to fill all of the available width (e.g. column's width). Card's height will be automatically resized to match width.

Here is an example on how to use it:

```html
<div class="card">
     
    <div class="card-height-indicator"></div>
     
    <div class="card-content">
         
        <div class="card-image">
            <img src="./image.jpg" alt="Loading image...">
            <h3 class="card-image-headline">Lorem Ipsum Dolor</h3>
        </div>
         
        <div class="card-body">
            <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry.</p>
        </div>
         
        <footer class="card-footer">
            <button class="btn btn-flat">Share</button>
            <button class="btn btn-flat btn-warning">Learn More</button>
        </footer>
         
    </div>
     
</div>
```      

Cards will adapt to column's width. The card below will have width equal to `col-lg-6`:
```html
<div class="row">
    <div class="col-lg-3"></div>

    <div class="col-lg-6">
        <div class="card">
            ...
        </div>
    </div>

    <div class="col-lg-3"></div>
</div>
```

For demo, visit [material.meteor.com](http://material.meteor.com) and scroll down to the 'Cards' section. Note that the flat buttons on that demo page are different from what we're having now. I will submit another PR for buttons later.

Note: I'm having trouble adding the click ripple effect on card's image. The class I added on [line 7](https://github.com/npvn/bootstrap-material-design/compare/FezVrasta:develop...card?expand=1#diff-ddd979003beabc725be6208094237874R7) of `material.js` doesn't seem to work.