Added support for storage flags in Codec

[thenobody]

Added setting and reading of the storage flags (as described in https://github.com/memcached/memcached/blob/master/doc/protocol.txt) into the public API. The intended point of usage is in the Codec trait's methods (i.e. serde operations should be aware of the flags, such as compression and similar).

Few notes:

• FakeMemcached internals don't support the flags so ignoring/setting 0s where needed
• unit test MemcachedSuite.increment-default is failing (it seems that it's been failing in previous builds as well)
• the default codecs in BaseCodecs accept but otherwise ignore the flag values

[lloydmeta]

It looks like #50 incorporates the flags on Codec idea.