Implemented the changelog to RPM

Implemented a way to import a changelog file to the generated RPM.
Also added a sample on the documentation of the rpm generation.

This was requested on issue #330.
Oh.. :( 
And what should I use instead of it?

> Em 26/08/2014, às 12:08, Nepomuk Seiler <notifications@github.com> escreveu:
> 
> In src/main/scala/com/typesafe/sbt/packager/rpm/RpmMetadata.scala:
> 
> > @@ -5,6 +5,7 @@ package rpm
> >  import linux.{ LinuxPackageMapping, LinuxFileMetaData }
> >  import sbt._
> >  import com.typesafe.sbt.packager.linux.LinuxSymlink
> > +import java.nio.file.{ Paths, Files }
> I overlooked this line. We are currently Java compatible, so you can't use NIO here :-(
> 
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