Makes prettyCheckable gracefully degradable

The below changes enables the user to write markup that degrades gracefully, if the prettCheckable js is switched off.

If markup is written like this (degradable):

    <label class="checkbox" for="example">
        <input type="checkbox"
               name="example"
               id="example"
               value="1"
               class="pretty"
               /><span>Some label text</span>
    </label>

PrettyCheckable will find 'Some label text', store it and then remove the span and use the label text.
In case prettyCheckable is not available, normal checkboxes will be rendered.


If the markup is written like this (non-degradable):

    <input type="checkbox"
           name="example"
           id="example"
           value="1"
           class="pretty"
           data-label="Some label text"
           />

The text from the data attribute will be used as it was before.

The only **important implication for the documentation** is, that the label text needs to be wrapped in a span element!