Enable autoloads

by add autoload cookies

Quote from [1] on the usage of 'autoload':
> the basic rule of thumb is to autoload any function which a user
> might reasonably want to use without necessarily require-ing the
> library first

And [2]:
> The general rule is to autoload interactive “entry points” of a package

[1]. http://de.reddit.com/r/emacs/comments/29jp7c/autoload_questions_what_functions_should_i/
[2]. http://www.lunaryorn.com/2014/07/02/autoloads-in-emacs-lisp.html

These above two link answered The following questions:
1. Why and when/how to use autoload
2. How to test if autoload is used