Add 'allow_unknown' option for nested fields

[otibsa]

I would like to have nested fields with arbitrary keys:

document = {
    'field1': {
        'nested1': 'foo'
        'arbitrary1': 'bar',
        'arbitrary2': 42
    }
}

schema = {
    'field1': {
        'type': 'dict',
        'allow_unknown': True,
        'schema': {
            'nested1': {'type': 'string'}
    }
}

allow_unknown can also be something like {'type': 'string'}.

[otibsa]

Ah.. by adding "schema" to special_rules, i might have killed some stuff in validate_schema(...), including #73

working on it

[nicolaiarocci]

Rebased and merged: f084f84

Thanks.