Make Close button optional and customizable

- Add option "closeText" ;
- Change close button's html to something more agnostic/semantic than an `a` tag with empty href ;
- Add creating close button only if pgwModal.config.close is true (prevents from having to hide it later) ;
- And bind click event if necessary.

New call may look like :
```
	$('#heroContact a.btn').on('click', function(e) {
		e.preventDefault();
		$.pgwModal({
			target: $(this).attr('href'),
			closeText: '<i class="icon-close"></i><span class="visuallyHidden">Close</span>',
			maxWidth: 800,
			loading: '<i class="icon-loader"></i><span class="visuallyHidden">Loading...</span>'
		});
	});
```