add simple rakefile for build tasks

Rakefile

+$LOAD_PATH.unshift File.expand_path("../lib", __FILE__)
+require "chef/handler/mail/version"
+
+GEM_NAME = "chef-handler-mail"
+GEM_VERSION = MailHandler::VERSION
+
+desc "Build #{GEM_NAME} gem"
+task :build do
+  system "gem build #{GEM_NAME}.gemspec"
+end
+
+desc "Push #{GEM_NAME} to rubygems.org"
+task :push => :build do
+  system "gem push #{GEM_NAME}-#{GEM_VERSION}.gem"
+end
+
+desc "Clean up build gems"
+task :clean do
+  system "rm #{GEM_NAME}-#{GEM_VERSION}.gem"
+end