adding spec_all task for running specs for all model adapters

Rakefile

@@ -7,4 +7,12 @@ RSpec::Core::RakeTask.new do |t|
   t.verbose = false
 end
 
+desc "Run specs for all adapters"
+task :spec_all do
+  %w[active_record data_mapper mongoid].each do |model_adapter|
+    puts "MODEL_ADAPTER = #{model_adapter}"
+    system "rake spec MODEL_ADAPTER=#{model_adapter}"
+  end
+end
+
 task :default => :spec