ScriptRunner: tweak javadoc

Now that the main method is the toplevel entry point,
the invoke method isn't anymore.
ScriptRunner: do some massaging of command args

In particular, it is nice if the Jython script can pass its own filename
(or partial file path) to ScriptRunner#main and have it be interpreted
as the correct ImageJ command, as long as the script is named properly.
ScriptRunner: improving timing messages

Now that the name of the command is determined semi-automatically, it
is good to have verification that the expected command is executing.