Added type as optional 2nd parameter to `array`.

Also added documentation in the format [used](https://github.com/JuliaStats/DataArrays.jl/blob/ae7d820/src/dataarray.jl#L260) in `DataArray`.

Should we add anything to handle NA's, or leave that off to `DataArray`? We might at least check for the presence of any NAs and give a more descriptive error than the current

```
ERROR: no method convert(Type{Float64}, NAtype)
 in setindex! at array.jl:342
 in array at /home/beyer/.julia/DataFrames/src/dataframe.jl:1451
```