Add support for gzip compressing SF data over-the-wire.

To enable gzip compression of responses from SF, just add a "gzip" property to the `opts` hash when creating the nforce connection.

Example:
```
var opts = { "gzip": true };
org = nforce.createConnection(opts);
```

If there is no `gzip` property explicitly set to boolean `true`, then no gzip compression is performed.

I've been using this in our production app for over a month now.  I've never seen performed adversely affected by this setting, and have seen gains of 2x or more when requesting more than a handful of records.