Update hadoop to install to libexec.

Install Hadoop to libexec, and then create shim scripts for the binaries
that run from the installed location.

This prevents PREFIX/lib from being spammed with  jars.

Formula/hadoop.rb

@@ -5,8 +5,27 @@ class Hadoop < Formula
   homepage 'http://hadoop.apache.org/common/'
   md5 '8f40198ed18bef28aeea1401ec536cb9'
 
+  def shim_script target
+    <<-EOS.undent
+      #!/usr/bin/env bash
+      cd #{libexec}/bin
+      ./#{target} $*
+    EOS
+  end
+
   def install
     rm_f Dir["bin/*.bat"]
-    prefix.install %w[bin conf contrib lib]
+    libexec.install %w[bin conf contrib lib]
+    bin.mkpath
+    Dir["#{libexec}/bin/*"].each do |b|
+      n = Pathname.new(b).basename
+      (bin+n).write shim_script(n)
+    end
+  end
+
+  def caveats
+    <<-EOS.undent
+      $JAVA_HOME must be set for Hadoop commands to work.
+    EOS
   end
 end