Issue 1324 calendar

Removing vendor/plugin calendar-date-select and adding gem.
Fix one of the three DEPRECATION WARNING: "You have Rails 2.3-style plugins in vendor/plugins"

After the installation of the gem, a new step in the installation should be insert to install calendar javascripts in public folder:   **`bundle exec rake calendar:install`**. Otherwise, it will not work. 

Source:http://stackoverflow.com/questions/3979495/using-calendar-date-select-with-rails-3