really make title optional + more custom close button

Don't create "title" html if no title is provided (pgwModal.config.title) in config (prevents having to hide it in CSS).

This is pretty much my first pull-request, hope I did things correctly...
Please disregard tabs/spaces diffs, the only interesting ones are lines 41-45, sorry about that.

ALSO, from later edit/commit for pull request:
"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>'
		});
	});
```
"

Thanks! :)