allow custom ID in Table.insert

Dropbox Datastore allows passing in an ID as a field.

Also, awesome mock library :)

```
> mocha src/test/js
  Datastore
    recordsChanged.addListener
      ✓ can be triggered by triggerRecordsChanged
    Record.get
      ✓ reads values from the mock data
    Record.update
      ✓ update values in the record
    Record.deleteRecord
      ✓ removes the record from the mock data
    Table.insert
      ✓ sets a unique id
    Table.insert
      ✓ set with custom id
    Table.get
      ✓ gets record by unique id
    Table.query
      ✓ queries all records
      ✓ queries with given properties

     9 passing (11ms)
```