add support for annotation only fields (#41)

* add support for annotation only fields, fix #34

* adding tests with mypy

* adding docs for mypy usage

* adding mypy failure test

* adding alias tests

* tweak mypy tests

docs/examples/example2.py

@@ -0,0 +1,30 @@
+from pydantic import ValidationError
+try:
+    User(signup_ts='broken', friends=[1, 2, 'not number'])
+except ValidationError as e:
+    print(e.json())
+
+"""
+{
+  "friends": [
+    {
+      "error_msg": "invalid literal for int() with base 10: 'not number'",
+      "error_type": "ValueError",
+      "index": 2,
+      "track": "int"
+    }
+  ],
+  "id": {
+    "error_msg": "field required",
+    "error_type": "Missing",
+    "index": null,
+    "track": null
+  },
+  "signup_ts": {
+    "error_msg": "Invalid datetime format",
+    "error_type": "ValueError",
+    "index": null,
+    "track": "datetime"
+  }
+}
+"""