Optional Cache Value Compression

[Jared314]

MongoDB has a 16MB document size limit, and, currently, does not have built in support for compression. So, I found it useful to gzip large Buffers I stored using cacheman-mongo.

var cache = new Cache({ compression: true }, {});
cache.set('test1', new Buffer("something big"), function (err) {...});

It only compresses Buffers because of the complexity in correctly decompressing and deserializing the variety of data structures and string encodings. (Using the BSON serializer might be a good solution to this problem, but I have not yet explored that option.)

[cayasso]

@Jared314 sorry, I totally missed this PR. cacheman-mongo@0.2.1 now has this feature, I couldn't merge it because 0.2.1 files changed and there is no longer a lib folder, but I copy the code just how it was and did some minor changes. Thanks for your contribution.