MongoDB ops appear to store redundant data

This is a small issue, but the MongoDB schema seems unnecessarily verbose:

``` javascript
{
opData: {
op: [
{
p: [
"text",
611
],
si: " "
}
],
meta: {
source: "b1c6063989dbca821fd9ded844031305",
ts: 1356560181754
}
},
_id: {
doc: "draft-text-b2aa543046d47579aad4738a",
v: 2
}
}
```

Perhaps the `opData` object could be flattened into the root object? This could be changed in a backwards compatible way by checking for the existence of the `opData` object.

(cc @zamfi - I think you might know the answer to this!)