Fixes tiny documentation error. Adds possibility to add custom validation methods to fields, e. g.:

class Customer(Document):
        country = StringField(validation=lambda value: value in ['DE', 'AT', 'CH'])

Replaced some str() with unicode() for i18n reasons.

docs/guide/defining-documents.rst

@@ -22,7 +22,7 @@ objects** as class attributes to the document class::
     
     class Page(Document):
         title = StringField(max_length=200, required=True)
-        date_modified = DateTimeField(default=datetime.now)
+        date_modified = DateTimeField(default=datetime.datetime.now)
 
 Fields
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