Add riakc_datatype:context/1 function

Context value of CRDT object could be useful for operations like calculating HTTP Etag. Currently, the only way to retrieve context is to deal with internal representation of CRDT object.

```erlang
{map,_,_,_,Context} = Obj.
```

By adding `riakc_datatype:context/1`, we could make such operations much more reliable.

```erlang
-callback context(datatype()) -> context().
```