additional convert methods for Nullable

Allows for : 
```
julia> type Bar
         a::Nullable{Int}
         b::Nullable{Bool}

         Bar() = new(nothing, nothing)
       end

julia> bar=Bar()
Bar(Nullable{Int64}(),Nullable{Bool}())

julia> bar.a=1
Nullable(1)

julia> bar.a=nothing
Nullable{Int64}()

```

EDIT: The PR has been updated to only support assignment of a non-null value to a Nullable field. See discussion below.