convert values to a function, fix #28 (#43)

docs/examples/exotic.py

@@ -37,7 +37,7 @@ m = Model(
     email_address='Samuel Colvin <s@muelcolvin.com >',
     email_and_name='Samuel Colvin <s@muelcolvin.com >',
 )
-print(m.values)
+print(m.values())
 """
 {
     'cos_function': <built-in function cos>,

docs/examples/recursive.py

@@ -17,5 +17,5 @@ class Spam(BaseModel):
 m = Spam(foo={'count': 4}, bars=[{'apple': 'x1'}, {'apple': 'x2'}])
 print(m)
 # > Spam foo=<Foo count=4 size=None> bars=[<Bar apple='x1' banana='y'>, <Bar apple='x2' banana='y'>]
-print(m.values)
+print(m.values())
 # {'foo': {'count': 4, 'size': None}, 'bars': [{'apple': 'x1', 'banana': 'y'}, {'apple': 'x2', 'banana': 'y'}]}