Add support for creating a RPM package with maven

Note: This has been disabled by default and is therefore not included in a standard build. The main reason for this is, that you need to have a RPM binary and the rpm development packages installed, which is not the case on many systems (especially mac os x).

I was able to install the RPM on opensuse and fedora and centos (using systemd to start elasticsearch on opensuse and the init script on centos). I would be glad if other people could test this as well.

You can build your own RPM package simply by running `maven rpm:rpm` in the source directory.