Add autoloading

[ashleyfae]

Enhancement Request

Explain your enhancement (please be detailed)

We should add some kind of auto loader so we can start loading classes automatically.

Justification or use case

We have an ever growing list of manual includes that 1) is hard to manage; and 2) makes the main EDD class a pain to read/navigate.

[rwebster85]

This would be welcomed. I like using the PSR 4 class based one, where you supply an array of sources to be registered upon instantiation.