Default to using maxSockets setting from https.globalAgent.

This change allows the SDK to inherit the maxSockets property
from the global agent in Node.js so that users do not have to
pass in a custom agent to increase maxSockets. Instead, this
change allows users to do the following to increase maxSockets:

```javascript
require('https').globalAgent.maxSockets = 20;

var s3 = new AWS.S3();
for (var i = 0; i < 20; i++) {
  s3.getObject(params, function (err, data) { /* ... */ });
}
```

Fixes #224