Adding BSONSymbol converter

Introducing a $symbol notation to handle the conversion between BSON and
JSON.

Also, as a note, on the ReactiveMongo snapshot I built against, it looks like the package location of `Converters` has changed from:

```scala
import reactivemongo.utils.Converters
```

to

```scala
import reactivemongo.bson.utils.Converters
```

This is why I updated the `bsonhandlers.scala` file. I can change this back if there is a specific snapshot I should have built against.

Also, I am open to using something besides $symbol, just let me know.

Best,
 -Eric