Improved extension interface

c.f. 8fc41d0, #1634 & #1635

**NOTE** requires a `composer update` in Bolt's root
This shouldn't be required, because is_readable is case-insensitve, even on filesystems that are. LOLPHP to the max. On my OSX machine `is_readable('REadmE.mD')` returns `true` for 'readme.md`, and i can even file_get_contents it. right? 

http://stackoverflow.com/questions/3964793/php-case-insensitive-version-of-file-exists