Nullable additions

I would like to add two convenience member functions to `Rcpp::Nullable<T>` specifically:

``` c++
T Nullable::as()
{
    return T(m_sexp);
}

T Nullable::clone() const
{
    return T(Rcpp:::clone(m_sexp));
}
```

These are analogous to the `Rcpp::as` and `Rcpp::clone` functions and just add convenience to the `Nullable` class.

Any objections?
