Add possibility to set card template with attribute string or with inner HTML instead of using an external template file

The card templates can now be provided by using a cardTemplateString attribute (which uses the attribute value as the template) or by omitting any template attributes (which uses the inner HTML of the directive as the template). Specs for these new features are included. This should fix both aspects of issue #7.

These examples (taken from the extended `README.md`) illustrate the new functionality:

`cardTemplateString` attribute:

    <div deckgrid source="photos" cardTemplateString="<p>{{card.title}}</p>"></div>


No template-related attribute:

    <div deckgrid source="photos">
        <div class="a-card">
            <h1>{{card.title}}</h1>

            <img src="" data-ng-src="{{card.src}}">
        </div>
    </div>