Add support for raw data flag

[rkday]

Hi,

I'm using python-binary-memcached to help test an application which already uses the 'opaque' field in memcached - which means that sometimes the right bit is set for python-binary-memcached to incorrectly interpret my data as compressed data or an integer, and fail to parse it.

This adds a raw_data flag, which maks it skip serialisation and deserialisation and just treat the data as binary.

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This change is 

[jaysonsantos]

Hey @rkday sorry for the long delay. I am mostly trying to maintain compatibility with the other libraries. Do you know what is the expected behavior for this in pylibmc for example? One suggestion is to run the dogpile cache's tests using your branch and see how it behaves, it has been a good way to check if compatibility is not broken.

[rkday]

In pylibmc the Client class can be subclassed to override the serialize and deserialize methods (http://sendapatch.se/projects/pylibmc/reference.html#pylibmc.Client.serialize) - maybe that would be a better way to do what I want.