Add pom.xml for publishing to sonatype / maven central

Add pom.xml for publishing to sonatype / maven central, refs #1

This version is deployed to sonatype and synced to maven central:
http://repo1.maven.org/maven2/com/esotericsoftware/kryonet/

Currently tests failing when running `mvn install`:

    Tests in error:
      testRMI(com.esotericsoftware.kryonet.rmi.RmiSendObjectTest): Address already in use

The full output of `mvn install` is available here:
https://dl.dropboxusercontent.com/u/1439289/mvn-install.log.zip

To be able to build the jar `mvn install` was run with `-Dmaven.test.skip=true`.