Load dygraph-combined when loading from NPM

Load dygraph-combined when loading from NPM

Today, when loading dygraphs from NPM via browserify you need to include:

```javascript
var Dygraph  = require('dygraphs/dygraph-combined.js');
```

Being able to do this:
```javascript
var Dygraph  = require('dygraphs');
```

Is preferable.