Fix Maven issues

[stelfrich]

The Travis build was failing for quite some time now. This was mainly due to Maven 2 to 3 migration issues and changes in the definition of Mojos (from JavaDoc annotations to "proper" annotations).

Inspired by scijava-maven-plugin, I have added annotations for the fields of all Mojos.

[ctrueden]

If we are using Java annotations, do we still need the javadoc annotations too?

[stelfrich]

If we are using Java annotations, do we still need the javadoc annotations too?

The plugin won't build with Maven 2 anyway (see below) due to the use of Maven 3 plugins. So I guess it is safe to remove the javadoc annotations... I'll go ahead and do it!

➜  imagej-maven-plugin git:(copy-jar-subdirectory) ~/Downloads/apache-maven-2.2.1/bin/mvn clean install
Warning: JAVA_HOME environment variable is not set.
[INFO] Scanning for projects...
Downloading: http://repo1.maven.org/maven2/org/codehaus/plexus/plexus-utils/1.1/plexus-utils-1.1.jar
164K downloaded  (plexus-utils-1.1.jar)
[INFO] ------------------------------------------------------------------------
[INFO] Building ImageJ Maven plugin
[INFO]    task-segment: [clean, install]
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Error resolving version for 'org.apache.maven.plugins:maven-clean-plugin': Plugin requires Maven version 3.0
[INFO] ------------------------------------------------------------------------
[INFO] For more information, run Maven with the -e switch
[INFO] ------------------------------------------------------------------------
[INFO] Total time: < 1 second
[INFO] Finished at: Tue Aug 22 06:57:25 CEST 2017
[INFO] Final Memory: 11M/481M
[INFO] ------------------------------------------------------------------------

[ctrueden]

Thank you very much!