Improvements to builtins min/max

[rmcgibbo]

Closes #77. I tested it with:

$ cat test.py
from typing import *
x = [1, 2, 3]

max(x)
max(x, key=lambda x:-x)
max(1,2,3, key=lambda x:-x)
max(x, default=1)
max(1,2,3, default=1)

min(x)
min(x, key=lambda x:-x)
min(1,2,3, key=lambda x:-x)
min(x, default=1)
min(1,2,3, default=1)

$ MYPYPATH=stdlib/3/ mypy test.py

[gvanrossum]

Thanks! The CI test is failing but I think that's not your fault.