add `errors` property and `is_valid` method to schema and validators
remove unused __from_string_excs__ attr
passing additional `default_registrar` and `default_getter` to sender's `add_listener` and `get_listeners`

This is to allow execution of default event registration/retrieval operations
when default action is overloaded
fix `add_listener` method and add `add_listener` method tests
add `alias_of` function to `formify.event` module

The function is used to mark one sender as "alias" of another, real sender, so
once event system related operations are done using aliased sender, all are
forwarded to real sender, unless alias does not allow to do so.
do not register same listener 2nd time
moved core validators to __init__.py and remove base.py