Fix some critical bugs in our Environ class

The two big ones were:
  * we weren't passing the 'my' argument to our parents when looking up
    values: we were just saying self._parent[key]
  * The cache idea in Environ was broken: since computed values depend on
    self and 'my', we can't get away with calculating them once per Environ,
    since they will differ depending on the value of 'my'.

This doctest example (also added) explains these bugs pretty well:

       >>> class Animal(Environ):
       ...    def __init__(self, p=None, **kw):
       ...       Environ.__init__(self, p, **kw)
       ...    def _get_limbs(self, my):
       ...       return my['legs'] + my['arms']
       >>> a = Animal(legs=2,arms=2)
       >>> spider = Environ(a, legs=8,arms=0)
       >>> squid = Environ(a, legs=0,arms=10)
       >>> squid['limbs']
       10
       >>> spider['limbs']
       8

If the first bug isn't fixed, both 'limbs' values say 4.  If the first
bug is fixed but the second is not, then both values say 10 (since the
10 gets cached.

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