Allow SDK to be be browserified without custom scripts

This patch allows the SDK to work with `browserify` out of the box. Consider the following application code that uses the SDK:

```js
// index.js
var AWS = require('aws-sdk');
var s3 = new AWS.S3;
```

Currently, this file would not work with:

```sh
$ browserify index.js > out.js
```

This patch allows the above command to succeed with the default SDK browser build. This patch *also* allows for custom services to be specified when browserifying via `AWS_SERVICES`:

```sh
$ AWS_SERVICES=ec2 browserify index.js > out.js
```

This relates to #383