Add ability to return raw values from the datastore (sans marshalling)

[nemtsov]

As a client configuring a new store,
I want to force distribucache to not marshall values,
In order to optimize for memory and cpu.

This is useful when you need to store a buffer or string and not an object (e.g., caching the html of the page). It will avoid the Buffer (Redis) -> String (Redis) -> Object (Distribucache) -> String (Express) -> Buffer (Http) conversion that currently happens with the Redis store when storing the page html in cache.

Possible configuration:

cache = cacheClient.create('page:v1', {
  useRawValues: true
});

[nemtsov]

As of v5.0.0, enabling optimizeForBuffers flag will skip marshalling. (https://github.com/dowjones/distribucache/blob/master/CHANGELOG.md#500---2015-06-01)