remove load_paths, lean Reloader on default prerequisites

This patch is a part of fixing #1688 

As I understand, initially load_paths was designed for Padrino to be able to reload files of an app. Then prerequisites was added and now load_paths partly duplicates default prerequisites of an app.

The second function of load_paths was to tamper with system variable $LOAD_PATH to tell Ruby which path can be looked at to find required file. It's not obvious to the user why some files are able to be required and some are not. I think if a user wants to require things manually and relatively, she should tamper with $LOAD_PATH herself.

In this patch I remove load_paths from Padrino core and improve the Reloader to track application prerequisites instead.

This patch will break some apps which are weird enough to remove `lib, shared, models, mailers, controllers, helpers` folders from dependencies and require them manually leaning on load_paths.