Fix #75 handleError issue with 404.html

This fixes the problem when trying to use `handleError: false` to fall-through while having a 404.html page in the root directory. 

It also adds the handleError option default in `opts.js`,  3 additional test files for Core, Express and Union; 
a `404.html` page to confirm correct behavior and updated documentation for the option in the README.

cheers! :octopus: