Adds support for proto2 syntax for Ruby gem.

This change only adds basic proto2 support without advanced features
like extensions, custom options, maps, etc.

The protoc binary now generates ruby code for proto2 syntax.
However, for now, it is restricted to proto2 files without advanced features
like extensions, in which case it still errors out.

This change also modifies the DSL to add proto messages to the DescriptorPool.
There is a new DSL Builder#add_file to create a new FileDescriptor. With this,
the generated ruby DSL looks something like:

Google::Protobuf::DescriptorPool.generated_pool.build do
  add_file "test.proto" do
    add_message "foo" do
      optional :val, :int32, 1
    end
  end
end

1 files changed, 21 insertions, 0 deletions

diff --git a/ruby/tests/generated_code_proto2_test.rb b/ruby/tests/generated_code_proto2_test.rb
new file mode 100644
index 00000000..e988abd0
--- /dev/null
+++ b/ruby/tests/generated_code_proto2_test.rb
@@ -0,0 +1,21 @@
+#!/usr/bin/ruby
+
+# generated_code.rb is in the same directory as this test.
+$LOAD_PATH.unshift(File.expand_path(File.dirname(__FILE__)))
+
+require 'generated_code_proto2_pb'
+require 'test_import_proto2_pb'
+require 'test_ruby_package_proto2_pb'
+require 'test/unit'
+
+class GeneratedCodeProto2Test < Test::Unit::TestCase
+  def test_generated_msg
+    # just test that we can instantiate the message. The purpose of this test
+    # is to ensure that the output of the code generator is valid Ruby and
+    # successfully creates message definitions and classes, not to test every
+    # aspect of the extension (basic.rb is for that).
+    m = A::B::Proto2::TestMessage.new()
+    m2 = FooBar::Proto2::TestImportedMessage.new()
+    m3 = A::B::Proto2::TestRubyPackageMessage.new()
+  end
+end