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Add Presence functionality

This change adds the ability for clients to broadcast information about
"Presence" - the notion of a client's position or state in a particular
document. This might be represent a cursor in a text document, or a
highlighted field in a more complex JSON document, or any other
transient, current information about a client that shouldn't necessarily
be stored in the document's chain of ops.

The main complication that this feature solves is the issue of keeping
presence correctly associated with the version of a `Doc` it was created
at. For example, in a "naive" implementation of presence, presence
information can arrive ahead of or behind ops, which - in a text-based
example - can cause the cursor to "jitter" around the change. Using the
ShareDB implementation will ensure that the presence is correctly
transformed against any ops, and will ensure that presence information
is always consistent with the version of the document. We also locally
transform existing presence, which should help to keep (static) remote
presence correctly positioned, independent of latency.

In order to facilitate this, the feature must be used with an OT type
that supports presence. The only requirement for supporting presence is
the support of a `transformPresence` method:

```javascript
type.transformPresence(presence, op, isOwnOperation): presence;
```

* `presence` _Object_: the presence data being transformed. The type
  will define this shape to be whatever is appropriate for the type.
* `op` _Op_: the operation against which to transform the presence
* `isOwnOperation`: _boolean_: whether the presence and the op have the
  same "owner". This information can be useful for some types to break
  ties when transforming a presence, for example as used in
  [`rich-text`][1]

This work is based on the [work][2] by @gkubisa and @curran, but with
the following aims:

  - avoid modifying the existing `Doc` class as much as possible, and
    instead use ... (continued)

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