Use non-infringing language for plugin name

We are supposed to call it the "NAR plugin for Maven" or
(less ideally but shorter) the "NAR plugin".

For details, see this message from the maven-users list:
http://mail-archives.apache.org/mod_mbox/maven-users/201306.mbox/%3CCA%2BnPnMzeORcpFL8gGq%3DycKFgSE-5WzBHM8PuvX-NZqOBEF6J0A%40mail.gmail.com%3E

The most relevant paragraph is:
> The allowed pattern of usage is that where the use of the mark is
> referring to the plugin being for use with Maven, hence "___ Plugin
> for Maven" is fine but "Maven ___ Plugin" is not. Obviously "___'s ___
> plugin for Maven" is even better, e.g. "Mojo's Cassandra plugin for
> Maven" (which has the bigger fun of using two Apache marks...)
Complete the 'Maven NAR' -> 'NAR' renaming

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Change groupId to com.github.maven-nar

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Rename the packages

Since '-' is not a valid part of a package name, we use
com.github.maven_nar instead of com.github.maven-nar, as per
http://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>