HelloWorld.jy: make various improvements

Most importantly, this change updates the ScriptRunner reference to
reflect the new package: imagej.omero rather than imagej.omero.server.

The script also does all the imports up front, shortening later
references using Python's "import foo as bar" syntax.

Lastly, the script now emits timestamps at the beginning and end of
execution, to aid in analysis of time performance.
Add temporary hack to ImageJ commands