[Command line] Add a command line option that prints out where compass is installed.

lib/compass/exec.rb

@@ -163,11 +163,15 @@ END
         end
 
         opts.on('--imports', :NONE, 'Emit an import path suitable for use with the Sass command-line tool.') do
-          #XXX cross platform support?
           print ::Compass::Frameworks::ALL.map{|f| "-I #{f.stylesheets_directory}"}.join(' ')
           exit
         end
 
+        opts.on('--install-dir', :NONE, 'Emit the location where compass is installed.') do
+          puts ::Compass.base_directory
+          exit
+        end
+
         opts.on('--validate', :NONE, 'Validate your project\'s compiled css. Requires Java.') do
           self.options[:command] = :validate_project
         end