feat(): added onPageRender callback method

when onPageRender method is defined on scope where directive is placed, it
is called when page of pdf is rendered, so we can disable next/previous
while rendering buttons because fast changing of pages causes rendering
two pages in same canvas

Author: matejhustava

dist/angular-pdf.js

@@ -40,7 +40,12 @@
               viewport: viewport
             };
 
-            page.render(renderContext);
+            var pageRendering = page.render(renderContext);
+            pageRendering.promise.then(function() {
+              if (typeof scope.onPageRender === 'function' ) {
+                scope.onPageRender();
+              }
+            });
           });
         };