* Change serialization interface to be an object

Rather than passing separate serialization and deserialization methods
to a pymemcache client, pass an object implementing a very simple
two-method interface.

This is a rather significant breaking change and should be part of an
x.0.0 major release.

Resolves #56

As suggested in that issue, this is a cleaner interface, as there's no
sensible context in which you would provide only one of these two
methods and it should therefore be thought of as a
serialization/deserialization protocol.

Also adds a note to the documentation's Best Practices list that you
should use the built-in serializer object unless you have a reason to do
otherwise.

* Support "de/serializer" in addition to "serde"

In order to support older client usage in addition to the new
serialization object (protocol), restore the "serializer" and
"deserializer" arguments to the Client classes.
These are marked as deprecated and will be automatically wrapped into a
small "serde" object.

In order to make the various object names more distinguishable and more
informative, the built-in default serializer is now called
"python_memcache_pickle_serde"

Additionally, default client.serde to a "no-op serializer".
This object does no transforms on the data. By putting this in place, we
can skip some conditionals in the code around presence or absence of a
serializer and therefore simplify internally (at the cost of an extra,
unnecessary, functional call in some cases).

It also simplifies logic around the handling of flags because we are now
*guaranteed* the presence of a serializer object which returns some
flags. i.e. "default flags" are no longer the responsibility of the
various serializer usage sites.
This is done carefully to ensure that passing a `serializer` without a
`deserializer` is respected.

When defining `long_description` in setup.cfg using the `file:`
directive, we need to keep everything on one line. Otherwise, it will
treat the value as a block of literal text.

resolves #239

Add a warning to the Best Practices section of the Getting Started
doc which explains the interaction between `noreply=True` and error
handling.

Also, fix a very minor typo, ``"noreply`` in a nearby line.

This updates the changelog and includes a final documentation cleanup
pass.

The client's `server` argument can now be a string containing the path
to the memcached server's UNIX domain socket.

Also document all of the connection-oriented client parameters.

With python3 (3.5 here) json_deserializer receives the values as bytes. 

Disclaimer: I am a python beginner, so if there is another way to achieve the same result please let me know.