Reworked the maven project to allow a Maven Central Release

This is a pull request for issue #12. The problem is described over at the issue page.

What this PR changes:

1. reformatted the the pom to have a more common order of the tags and added spacing for readability.
2. removed all warning by maven due to missing plugin versions and source encoding.
3. changed the information to confirm to the sonatype-release requirements:
    - added the developers tag: the names were taken from the README and the role is Contributer
    - replaced the repositories in distributionManagement by the sonatype repositories as per requirement. The configured deploy plugin version allows to override the repositories through command line arguments if this is necessary for a CI environment.
    - added the sonatype-oss parent as per requirement
    - changed the scm management to point to github
    - added a profile called "release" that configures both the gpg plugin for code signing (sonatype requirement) and the release plugin for releases.