Registering new resource after Eve's init

New method Eve.register_resource() for registering new resource after initialization of Eve object. This is needed for simpler initialization API of all ORM/ODM extensions. Example of this api: https://github.com/hellerstanislav/eve-mongoengine/tree/init-api-refactor

Added minor changes - i.e. moved supported methods to class attribute and new property api_prefix.

Maybe we can break `_set_resource_defaults()` into smaller methods with semantically appropriate names (i.e caching_defaults, query_defaults, system_defaults etc.)