Use index.html instead of example-list.html

The original decision to avoid index.html was because it only works with a build step.  Since we've all accepted a build step, the more sensible name is index.html.  Without this change, every hosted example has a link to http://openlayers.github.com/ol3/master/examples/example-list.html which 404s.

bin/example-screenshot.js

@@ -42,7 +42,7 @@
       exampleDir = system.args[2],
       // various settings
       ignoreFiles = [
-        'example-list.html'
+        'index.html'
       ],
       intervalMillisecs = 25,
       renderMillisecs = 2000,