Add default rake task, add .idea to .gitignore

Add a default rake task to Rakefile (The test task) and add .idea to the
.gitignore file, to allow JetBrains webstorm
(http://www.jetbrains.com/webstorm) and JetBrains RubyMine
(http://www.jetbrains.com/ruby) users to commit without having to delete
build files.