Better validators (#97)

* working on improved validators

* full tests for validators

* tweask

* tweaking fields.py

* adding docs

* add history

* fix classmethod validators

docs/examples/validators_simple.py

@@ -0,0 +1,35 @@
+from pydantic import BaseModel, ValidationError, validator
+
+
+class UserModel(BaseModel):
+    name: str
+    password1: str
+    password2: str
+
+    @validator('name')
+    def name_must_contain_space(cls, v):
+        if ' ' not in v:
+            raise ValueError('must contain a space')
+        return v.title()
+
+    @validator('password2')
+    def passwords_match(cls, v, values, **kwargs):
+        if 'password1' in values and v != values['password1']:
+            raise ValueError('passwords do not match')
+        return v
+
+
+print(UserModel(name='samuel colvin', password1='zxcvbn', password2='zxcvbn'))
+# > UserModel name='Samuel Colvin' password1='zxcvbn' password2='zxcvbn'
+
+try:
+    UserModel(name='samuel', password1='zxcvbn', password2='zxcvbn2')
+except ValidationError as e:
+    print(e)
+"""
+2 errors validating input
+name:
+  must contain a space (error_type=ValueError track=str)
+password2:
+  passwords do not match (error_type=ValueError track=str)
+"""