(optionally) attach to proxy model instead of concrete model

[gutard]

Currently, in case of proxy model, paperclip attach files to the concrete model content type and store it in a subdirectory with concrete model name. We may want to use proxy model content type and directory name instead.

This is related to https://code.djangoproject.com/ticket/17648.

We should provide a "FOR_CONCRETE_MODEL" setting and use it as a parameter to content type get_for_model() calls. We should provide a command to migrate data in case of setting change.