Use json file for hub config.

I mimicked the same functionality as the node config files. I tried to keep it plural for the future, if you wanted multiple hubs.

A few commented out lines can probably be deleted.

Also, look at GridHubConfiguration  : 
 ArrayList<String> servlets = new ArrayList<String>() {{
 add("com.groupon.seleniumgridextras.grid.servlets.SeleniumGridExtrasServlet");
 add("com.groupon.seleniumgridextras.grid.servlets.ProxyStatusJsonServlet");
 }};
I wasn't 100% sure this should be an ArrayList.

It seems to work from the minimal tests I've done though.