Issue 115

Added a StatusCodeEnum consistent with the Memcached protocol (and SASL auth extensions).  Added code throughout to convert int status codes to the enum.  Added a NodeNotFound status code, and return it when a node cannot be located.

ref [Issue #115](https://github.com/enyim/EnyimMemcached/issues/115)

Signed-off-by: Rob Mayer <rmayer@stewartstrategy.com>