Support per-request scope for fixtures

Originally reported by: BitBucket: brettatoms, GitHub: brettatoms

• Bitbucket: https://bitbucket.org/pytest-dev/pytest/issue/456

I'm not sure how to really describe this but here's a code sample that demonstrates the issue. Basically if I create a test that takes two fixtures as parameters and one of those fixtures depends on the other fixture then the fixture only gets called once and the test method has the same instance for both fixtures.

import sys
import pytest

class User:
    def __init__(self, username, is_admin=False):
        self.username = username
        self.is_admin = is_admin

@pytest.fixture
def user():
    return User('me')

@pytest.fixture
def admin_user(user):
    user.username = 'admin'
    user.is_admin = True
    return user

def test_me(user, admin_user):
    print('repr(user): ', repr(user))
    print('repr(admin_user): ', repr(admin_user))

Start by running the fixture example in the issue and tracing pytest's fixture dependency and scope handling; no source files or tests are named in the report. Done means a test with user and admin_user receives separate per-request instances while preserving fixture dependency behavior.