Validators for dataclasses (#454)

* Added validators for dataclass, fix #415

* Added dataclass validators

* Added dataclass validators

* Updated docs for added validating to dataclass

* Updated docs for added validating to dataclass

* Fixed line endings

* Set __validators__ type to Mapping instead of Dict

* Update History

* Use __mro__ instead of __bases__ for gather_validators

* Fix PR number

* Fix issue.rst header underline

* Fix HISTORY.rst merge conflict

* Fix utils.py merge conflict

* fix utils.py

* Rebase and other fixes

* Fix rebase and other issues

* Change history

* Remove unnecessary lines in main.py

* Rebase

* Update history

* Rename ModelType to ModelOrDc

* Added inheritance replace test

* More consiste dataclass validator tests

* fix history.

* Remove Optional ModelOrDc Type

* Fix ModelOrDc

docs/examples/datetime_example.py

@@ -0,0 +1,22 @@
+from datetime import date, datetime, time, timedelta
+from pydantic import BaseModel
+
+class Model(BaseModel):
+    d: date = None
+    dt: datetime = None
+    t: time = None
+    td: timedelta = None
+
+
+m = Model(
+    d=1966280412345.6789,
+    dt='2032-04-23T10:20:30.400+02:30',
+    t=time(4, 8, 16),
+    td='P3DT12H30M5S'
+)
+
+print(m.dict())
+# > {'d': datetime.date(2032, 4, 22),
+# 'dt': datetime.datetime(2032, 4, 23, 10, 20, 30, 400000, tzinfo=datetime.timezone(datetime.timedelta(seconds=9000))),
+# 't': datetime.time(4, 8, 16),
+# 'td': datetime.timedelta(days=3, seconds=45005)}