Update jython

* Add HEAD version for 2.5.2 beta
* Install to libexec and symlink jython script
* Cache the installer jar

Formula/jython.rb

@@ -1,12 +1,16 @@
 require 'formula'
 
 class Jython <Formula
-  JAR = 'jython_installer-2.5.1.jar'
-  url "http://downloads.sourceforge.net/project/jython/jython/2.5.1/#{JAR}"
-  homepage 'http://www.jython.org'
+  url "http://downloads.sourceforge.net/project/jython/jython/2.5.1/jython_installer-2.5.1.jar",
+    :using => :nounzip
   md5 '2ee978eff4306b23753b3fe9d7af5b37'
+  homepage 'http://www.jython.org'
+  head "http://downloads.sourceforge.net/project/jython/jython-dev/2.5.2b1/jython_installer-2.5.2b1.jar",
+    :using => :nounzip
 
   def install
-    system "java", "-jar", JAR, "-s", "-d", prefix
+    system "java", "-jar", Pathname.new(@url).basename, "-s", "-d", libexec
+    bin.mkpath
+    ln_s libexec+'bin/jython', bin
   end
 end