POM: use pom-scijava as parent

This is more consistent with the rest of the ImageJ projects. It also
avoids having a snapshot as parent, which prevents reproducible builds.
POM: simplify repositories block
POM: use GitHub username for developer ID
POM: remove worthless picUrl
POM: add license-maven-plugin configuration block
Update license and file headers

This was done using:
  mvn license:update-project-license license:update-file-header

The only real change is the loss of the legal disclaimer at the bottom
of the license. This disclaimer is not actually part of the BSD-2
license proper, but rather an addendum. For relevant discussion, see:
  https://jira.codehaus.org/browse/MLICENSE-80