Remove maven.imagej.net entries from pom-scijava

... in preparation for deploying to OSS Sonatype (which does not
allow external references to random Maven repositories).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
pom-scijava: Add a profile for deploying to OSS Sonatype

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
pom-scijava: Add the OSS Sonatype repositories

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Use the nexus-staging-maven-plugin for deploying

As described in

http://blog.sonatype.com/2013/09/simplified-releases-to-the-central-repository-with-nexus/

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