Add a health checker service implementation.

- adds the code-generated health service classes to the pb along with
  a README explaining how to regenerate the generated code

- adds an implementation of the Health Checker Service along with unit
  tests and an integration test

Also:
- adds a pb folder
  : in a follow-up PR, all ruby pbs + generated code will be moved to it
diff --git a/src/ruby/grpc.gemspec b/src/ruby/grpc.gemspec
index eb74845..22fafe1 100755
--- a/src/ruby/grpc.gemspec
+++ b/src/ruby/grpc.gemspec
@@ -24,7 +24,7 @@
   %w(math noproto).each do |b|
     s.executables += ["#{b}_client.rb", "#{b}_server.rb"]
   end
-  s.require_paths = %w( bin lib )
+  s.require_paths = %w( bin lib pb )
   s.platform      = Gem::Platform::RUBY
 
   s.add_dependency 'google-protobuf', '~> 3.0.0alpha.1.1'