Improve XML parsing with variable substitution

[wcmatthysen]

We can improve the XML parsing logic in WorldWind so that we can have some rudimentary features such as property-substitution. There are various scenarios where this can be an advantage. The original pull-request (located at: NASAWorldWind#22) describes a scenario where we have a server that serves imagery or elevation data and the server-name can be configured as a Java system property. Everywhere the server is referenced in one of the layer XML files we use something like ${map-server-name} and let WorldWind do the replacement internally based on what the value of that property is. We can have a look at what @rougeSE has done in this pull request as a starting point.