Support mixins in (Embedded)Document definition

[lafrech]

Closes #188.

This PR allows to use Mixin classes in Document and EmbeddedDocument definition.

It only works if the Mixin is declared first (leftmost) in the Document definition.

class MyMixin:
    a = fields.IntField()

class MyDoc(MyMixin, Document):  # Mixin first
    b = fields.IntField()

[coveralls]

Coverage increased (+0.006%) to 95.695% when pulling c9a8664 on mixins into 2f3322c on master.