Add pom.xml for publishing to Maven Central

Almost all Java dependencies are available through maven central and it would be useful to have czml-writer published there as well. I've added a pom that works with the current source tree.

There are still a few things that the project owner has to do.[1] 
* review group, artifact, and version
* publish cs2j.jar on maven central
* publish czml-writer on maven central

[1] https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide