Added concept of response backends to image resizer, so it's now trivial to use something like X-Sendfile and X-Accel-Redirect rather than serving the image data directly through the app.
Moved exceptions into their own module to avoid circular imports
Changes: Image Resizer - Refactored code slightly to split mods and cache into separate modules. Added support for overriding installed mods and cache backend with settings rather than subclassing ImageModifier and BaseImageHandler