Changing relations class_name to string

As o Rails documentation, these values must be string.
This causes a eager load of some files in order to resolve constants
Removing manual load models in favor of auto_load

Some rails gems like (active_admin, rails_admin and others) tries
to figure out the available models by traversing load paths. This adds merit models to
rails autoload paths, so they are now 'findable'.

To load shared models, each active record model should know how to load
it's own dependencies (once they are not autoloaded by ORM path).
Removind folder lib/merit/models/base/merit

There is no namespace Merit::Base::Merit::BadgesSach, the correct
one would be Merit::Base::BadgesSash, so this folder is unneeded and
leads to path confusion.
Merge pull request #146 from emoreth/fix_loading_paths

Fix loading paths