Use maven-shade-plugin to produce executable jar

Switch from using the maven-assembly-plugin to the recommended maven-shade-plugin for creating the executable (i.e. dependencies included) jar.  The build now produces both

1) ttorrent-VERSION.jar without dependencies suitable for deployment to maven repositories and subsequent use as a library dependency in other maven projects.

2) ttorrent-VERSION-shaded.jar which includes all dependencies and a Main-Class manifest attribute allowing it to be executed at the command-line.  This jar is built but not "attached" to the project at runtime, which means it won't be included in the set of artifacts uploaded by the "install" or "deploy" targets.