Rename the "document_id" field to "d_id"

Hey Aaron. This is probably going to be a contentious pull request, because it's not backwards-compatible. Nonetheless, for large data sets, it's a simple, yet useful, improvement.

Ideally, each of the fields would be a single character. For example, instead of this:

```
{ "_id" : ObjectId("4e8d2efb2b58af7d4c00000a"), "ngram" : "ebe", "d_id" : ObjectId("4e6299b6349c301b2900000d"), "score" : 0.4714045207910317, "class" : "Product" }
```

it'd be something like this:

```
{ "_id" : ObjectId("4e8d2efb2b58af7d4c00000a"), "n" : "ebe", "d" : ObjectId("4e6299b6349c301b2900000d"), "s" : 0.4714045207910317, "c" : "Product" }
```