Added setindex! for SubDataFrames

Before:

```julia
julia> g[(g["sample_id"] .== sample_id[i]),"parental_id"] = sample_id[i-1]
ERROR: no method setindex!(SubDataFrame,ASCIIString,DataArray{Bool,1},ASCIIString)
```

After:
```julia
g[(g["sample_id"] .== sample_id[i]),"parental_id"] = sample_id[i-1]
"TCGA-02-0003-10A"
```

This is a simple, one-liner.