allow more flexible template for limit

[rufinus]

I needed to overwrite your base template to allow select2 filter boxes i got some problems with the positioning, and needed a surrounding div.
After that change the onChange trigger not longer works as it is very restrictive where it searches for the select element.

this pull request tries to relax this a little.

Example of my usecase:

{% extends 'EMCTableBundle::template.html.twig' %}

{% block tfoot %}
    <tr>
        <td colspan="{{ thead|length + (allow_select ? 1 : 0) }}">
            {{ block('pages') }}
            <div class="pull-right">{{ block('limit_select') }}</div>
        </td>
    </tr>
{% endblock %}

{% block limit_select %}
    {% if total > limit %}
        <select id="{{ id }}_limit" class="select2 btn-xs limit" data-width="70px">
            <option value="10">10</option>
            <option value="20">20</option>
            <option value="50">50</option>
            <option value="100">100</option>
            <option value="0">{{ 'All'|trans({}, 'EMCTableBundle') }}</option>
        </select>
    {% endif %}
{% endblock %}