#178 GroovyShell plug-in: use the groovysh script to start a Groovy shell

[Evgeny Roubinchtein]

GroovyShell plugin appears to rely on a rather old version of Groovy; it also failed to actually start a Groovy shell in my set-up. The attached patch uses the groovysh script/wrapper to start the Groovy shell. It appears to work for me on a Linux box (Ubuntu 12.04 with tuxjdk 1.8u66).