Refactored for autoload and Composer support

For https://github.com/markjaquith/WP-TLC-Transients/issues/16 Went ahead and did refactoring for example. Left in `require`s for non-Composer use.

Example standalone install for the branch:

    composer create-project markjaquith/wp-tlc-transients tlc-transients dev-composer --repository-url=http://rarst.net