Minc2volume viewer export functions

The `minc2volume-viewer.js` script is now able to export two javascript functions when it is `require()`d.

The original use case stays the same:
```bash
  $ node minc2volume-viewer.js <filename>
```

But we can now use it as a module:
```javascript
var minc2volumeViewer = require('./minc2volume-viewer')
minc2volumeViewer.getHeader('<filename>', function (err, header) {
  ...
})
var rawStream = minc2volumeViewer.getRawDataStream('<filename>')
rawStream.pipe(...)
rawStream.on('error', function (err) { ... })
```