This makes MultiDictView work with a simple getter/setter pair, rather than
using attributes with implicit leading underscores. Also move MultiDictView
into multidict.py and adds some simple unit tests.
This commit introduces MultiDict, a multi-dictionary similar to
ODict, but with improved semantics (as in the Headers class).
MultiDict fixes a few issues that were present in the Request/Response
API. In particular, `request.cookies["foo"] = "bar"` has previously been a
no-op, as the cookies property returned a mutable _copy_ of the cookies.
