Libraries should raise library specific exceptions. Such as
MemcacheError. This enables users to easily catch exceptions generated
by library separate from other exceptions. Raising "Exception" forces
catching Exception and potentially masks all sorts of issues.

Also wrap a line that exceeds 80 characters.

The documentation for `ignore_exc` says it will treat memcache/network
errors as cache misses (i.e. `None` for reads and `False` for writes).

However the short-circuit path for when no client can be used (because
they're all marked as down/bad) always returns `False`. This change
change the behaviour to return each command's default value instead.

I actually stumbled upon this because I wanted to inspect some behaviour
in a system without cache so I stopped all memcached nodes. Our code
looked something like this:

    data = memcached_client.get(key)
    if data is None:
        return fetch_from_database(key)
    else:
        return json.loads(data)

What happened was that every once in a while I would see a stacktrace
because the `json.loads(data)` call would fail with a `TypeError`
because it tries to deserialize a bool (`False`).

memcached's ASCII protocol supports unicode keys, so lets support them
as well. Since using unicode keys for memcache is uncommon and to
preserve the previous behavior disable support by default.