Add default CSS style for mouse position

Missing CSS was confirmed with issue #680, this commit is to fix it and change
ol-mouse-position class to ol-mouseposition. I choose for the moment the top
right corner to display the coordinates from mouse position control because of
potential conflict with the scaleline control.

examples/mouse-position.js

@@ -13,6 +13,8 @@ var map = new ol.Map({
     new ol.control.MousePosition({
       coordinateFormat: ol.coordinate.toStringHDMS,
       projection: 'EPSG:4326',
+      // comment the following line to have the mouse position
+      // be placed within the map.
       target: document.getElementById('mouse-position'),
       undefinedHTML: ' '
     })