updated module to be able to deal with changes in download archive structure at sonatype ...

... and improved README example puppet manifest:
- as distribution only jre and jdk are allowed when using puppetlabs/java
- unified usage of variables $nexus_group and $nexus_user in README example
- changed default $revision to undef ($revision is used to compose the download url)
- added $revsion_in_archive to support archives such as nexus-2.7.0-bundle-tar.gz, in which the root directory is not nexus-2.7.0 but nexus-2.7.0-06 (see http://www.sonatype.org/nexus/archived)