fix issue with configuring options that are booleans

now debug_mode won't get stuck to the "on" position
numerous fixes and refactorings

- fixes issue with subfolders in assets dir
- uses image/jpeg instead of image/jpg content type for *.jpg files
- adds support for *.jpeg and *.webp images
- uses settings to DRY up middleware
- clean up sprockets-helpers integration a bit
updates specs and cleans up the code some more
Bump to 0.2.0