Update css and js for bootstrap 3 input groups

In version v3.1.1 of bootstrap the input-append and input-prepend have been replaced with input-group on the div and input-group-addon on the span.
For example:

```
<div class="input-group date" id="dateOfCorrespondence" data-date="${dateOfCorrespondence}" data-date-format="dd/mm/yyyy">
    <input type="text" class="form-control" value="${dateOfCorrespondence}" readonly />
    <span class="input-group-addon"><i class="fa fa-calendar"></i></span>
</div>
```

I have also updated the icons within the calendar to use font awesome but that's just personal preference so I understand if you do not wish to include that change.
```
headTemplate: '<thead>'+
    '<tr>'+
    '<th class="prev"><i class="fa fa-angle-double-left"/></th>'+
    '<th colspan="5" class="switch"></th>'+
    '<th class="next"><i class="fa fa-angle-double-right"/></th>'+
    '</tr>'+
'</thead>',
```