Add simple "Loading..." message to post list

It can take several seconds for the posts to display (due to the large number of them), so this adds a simple loading indicator.
![FireBlog Loading Indicator](http://i.dstaley.com/FireBlog_2014-09-30_17-34-41.jpg)

You can demo this pull request [here](http://i.dstaley.com/blog-lato/#/posts).