Bump stable and tested versions to 1.0.4 and 3.9.0 respectively.

Also:
* Improve inline documentation
* Minor refactoring.


git-svn-id: https://plugins.svn.wordpress.org/enable-image-scaling-option-on-upload/trunk@898207 b8457f37-d9ea-0310-8a92-e5e31aec5664