Add Style enforcer (via Rubocop)

It will fail the build, but as it is currently,
most of the cops are 'todos'. Great for new contributors.. :)

Rakefile

@@ -1,9 +1,22 @@
-require "bundler/gem_tasks"
+require 'bundler/gem_tasks'
+
+begin
+  require 'rubocop'
+  require 'rubocop/rake_task'
+rescue LoadError
+else
+  Rake::Task[:rubocop].clear if Rake::Task.task_defined?(:rubocop)
+  desc 'Execute rubocop'
+  RuboCop::RakeTask.new(:rubocop) do |task|
+    task.options = ['--rails', '--display-cop-names', '--display-style-guide']
+    task.fail_on_error = true
+  end
+end
 
 require 'rake/testtask'
 
 Rake::TestTask.new do |t|
-  t.libs << "test"
+  t.libs << 'test'
   t.test_files = FileList['test/**/*_test.rb']
   t.ruby_opts = ['-r./test/test_helper.rb']
   t.verbose = true