Add the capability to specify a Java system property in the web.xml file, and then use that property to override configuration.

This is a nice because then I can drop this webapp into multiple application containers and only change the system property for configuration instead of bundling it all in the WAR.
I forgot to add a not null check after I added the "file:///" prefix to make the URL() constructor call in the parser happy.
Merge pull request #59 from rylarson/master

Add the ability to specify the name of a Java System property for configuration