forward-port CRC32 code from old version

For a variety of reasons, the code I'm working on is dependent on an old version of node-memcached. Unfortunately, in versions prior to 0.1.1, node-memcached's package.json does not specify a version of hashring. In the default configuration in these older versions, node-memcached uses 'crc32' as it's hashing algorithm option. Unfortunately, support for this seems to have been dropped in hashring v1.0.0. Now we're in a state where new installs of any version of node-memcached prior to 0.1.1 are broken. 

For example, instantiating a Memcache object (at v0.0.7) using "localhost:11211" as a parameter causes the following:

<pre>
crypto.js:132
  return new Hash(hash);
         ^
Error: Digest method not supported
    at Object.exports.createHash (crypto.js:132:10)
    at HashRing.hash (/Users/bengl/memcachedtest/node_modules/memcached/node_modules/hashring/index.js:199:17)
    at HashRing.digest (/Users/bengl/memcachedtest/node_modules/memcached/node_modules/hashring/index.js:210:15)
    at each (/Users/bengl/memcachedtest/node_modules/memcached/node_modules/hashring/index.js:112:16)
    at Array.forEach (native)
    at HashRing.generate [as continuum] (/Users/bengl/memcachedtest/node_modules/memcached/node_modules/hashring/index.js:100:11)
    at new HashRing (/Users/bengl/memcachedtest/node_modules/memcached/node_modules/hashring/index.js:77:8)
    at EventEmitter.Client (/Users/bengl/memcachedtest/node_modules/memcached/lib/memcached.js:68:19)
    at Object.<anonymous> (/Users/bengl/memcachedtest/index.js:2:10)
    at Module._compile (module.js:449:26)
</pre>

We dealt with this internally by passing around a tarball of our app with the versions of everything frozen, but we'd like to be able to depend on npm instead.

In the spirit of ensuring older versions are not broken forever, I see two options:

1. Do a force publish on the older versions of node-memcached to npm, adding a version of 0.0.x for hashring to the package.json.
2. Re-implement the missing hash algorithm in the new code.

Option 1 might be the easiest, and that way you could worry a little less about backward compatibility issues. I'm generally a little timid about doing a force publish, but in this case I think it might be worth it, since the change would be so small and the fix would be tremendously helpful. I'd be perfectly happy to blow away my node_modules directory if it means this problem goes away. :)

Since option 2 may or may not be safer, I've gone ahead and done just that in this pull-request, in case you want to go down that route instead.