Ataki/gridfs

@stvstnfrd to review commit
Since this conversion factor is constant (`1000 * 1000`), I'd prefer to
not perform the calculation repeatedly. What about something like:
```javascript
// Somewhere near the top
var CONVERSION_FACTOR_FOR_MB_TO_B = 1000000; // 1000 * 1000
// ... further down ...
    this.uploadChunkFileSizeInBytes = this.uploadChunkFileSizeInMbs * CONVERSION_FACTOR_FOR_MB_TO_B;
    this.maxFileSizeInBytes = this.maxFileSizeInMbs * CONVERSION_FACTOR_FOR_MB_TO_B;
```

Since we're already using such an explicit naming convention for units,
_I_ feel like this is self-documenting (with a small perf. boost).

Thoughts?