Issue1011

Create an ant task to build a launcher script (e.g., java
-Dpython.executable <foo> -jar dist/jython.jar xxx) for Unix and Windows
platforms.  The script can be output in the dist directory along with
the built jar file.

This could also be used to run the regression tests via ant regrtest
(perhaps creating the script in a temporary directory), so those tests
which fail because sys.executable is not set will work.

This ticket is almost done -- Frank stole JRuby's launcher, now we have 
one living under dist/bin/jython. Nick also added some things to it

What's left to do before closing this out is create a Windows version

Before this can replace the 'jython' script generated by the installer we 
also need to prefer "jython-complete.jar" as "jython.jar" does not exist 
in the built version, and pass through the JAVA_HOME selected in the 
installer.

An initial Windows launcher script (has fewer features than the Unix
one) is in r4950.

Good enough for now.