Added support for multiple selection

I've added support for binding multiple checkboxes to the same model. You can enable this by adding the attribute "multiple" to the relevant checkboxes.

Example:
<pre><code>&lt;pretty-checkbox multiple label="'One'" ng-model="foo" value="'one'"&gt;&lt;/pretty-checkbox&gt;
&lt;pretty-checkbox multiple label="'One'" ng-model="foo" value="'one'"&gt;&lt;/pretty-checkbox&gt;
&lt;pretty-checkbox multiple label="'One'" ng-model="foo" value="'one'"&gt;&lt;/pretty-checkbox&gt;
</code></pre>

If you select the first two, the value of <code>$scope.foo</code> would be <code>["one","two"]</code>.