New API

[Bobronium]

Let's discuss new vaa api here.

Previously discussed:

• Make errors list of ErrorObj (Should we make it raisable?);
• Drop special types for validators in favor of functions and typing.NamedTuple. (done in #3)

I also suggest to:

• drop is_valid property and threat data as valid if no errors returned;
• rename cleaned_data to data (cause it shorter) and just assume that is valid (still not sure about this).

New api example:

validator = vaa.get_from(some_scheme)
some_data = {'a': 1}
result = validator(some_data)
result.data
result.errors
# or
data, errors = validator(some_data)

if errors:  # not valid
    ...

To think about:

• raising exception on errors?
• if we return errors and data is valid, should we return errors as a empty list or as None (same goes for data)?