PROV-613 : Let application plugins define their own css & js

See http://www.collectiveaccess.org/support/forum#/discussion/comment/310926 for an initial description of the issue. This is obviously not a showstopper but it does talk towards having fully self contained plugins.

Syntax to use in pluginnamePlugin.php :
```
		public function hookJavascriptCssLoadManager($pa_javascript_css_list) {
			$pa_javascript_css_list[] = "/museesDeFrance/css/test.css";
			$pa_javascript_css_list[] = "//js/test.js";
			$pa_javascript_css_list[] = "http://underscorejs.org/underscore-min.js";

			return $pa_javascript_css_list;
		}
```