Rewrite CaseInsensitiveDict to work correctly/sanely

Fixes #649 and #1329 by making Session.headers a CaseInsensitiveDict,
and fixing the implementation of CID. Credit for the brilliant idea
to map `lowercased_key -> (cased_key, mapped_value)` goes to
@gazpachoking, thanks a bunch.

Changes from original implementation of CaseInsensitiveDict:

1.  CID is rewritten as a subclass of `collections.MutableMapping`.
2.  CID remembers the case of the last-set key, but `__setitem__`
    and `__delitem__` will handle keys without respect to case.
3.  CID returns the key case as remembered for the `keys`, `items`,
    and `__iter__` methods.
4.  Query operations (`__getitem__` and `__contains__`) are done in
    a case-insensitive manner: `cid['foo']` and `cid['FOO']` will
    return the same value.
5.  The constructor as well as `update` and `__eq__` have undefined
    behavior when given multiple keys that have the same `lower()`.
6.  The new method `lower_items` is like `iteritems`, but keys are
    all lowercased.
7.  CID raises `KeyError` for `__getitem__` as normal dicts do. The
    old implementation returned `None`
6.  The `__repr__` now makes it obvious that it's not a normal dict.

See PR #1333 for the discussions that lead up to this implementation