Fix getindex() type ambiguity in DataArray

This fixes the error

```
Warning: New definition 
    getindex(DataArray{T,N},Union(Array{T,1},Ranges{T})) at /home/bork/.julia/DataFrames/src/dataarray.jl:348
is ambiguous with 
    getindex(DataArray{T<:Number,N},Union(Array{T,1},Ranges{T},BitArray{1})) at /home/bork/.julia/DataFrames/src/dataarray.jl:332.
Make sure 
    getindex(DataArray{T<:Number,N},Union(Array{T,1},Ranges{T}))
is defined first.
Warning: New definition 
```

from #373

I'm still learning the type system a bit, but I think this is right --

`DataArray(["abc", "def"])[BitVector(2)]` comes out to the right thing