Updates to ambiguous warning output

* Re-added source line numbers for new ambiguous definition.  These existed previously, but were lost in some merge.
* Cleaned up formatting

Old (minimal line wrapping, no line number on new def):

```julia
julia> using DataFrames
Warning: New definition getindex(DataArray{T<:Number,N},Union(Ranges{T},BitArray{1},Array{T,1})) is ambiguous with getindex(DataArray{T,N},Union(Array{Bool,1},BitArray{1})) at /scratch1/tmp/kmsquire/.julia/DataFrames/src/dataarray.jl:337.
         Make sure getindex(DataArray{T<:Number,N},Union(Array{Bool,1},BitArray{1})) is defined first.
```

New (more line wrapping, line number added)

```julia
julia> using DataFrames
Warning: New definition 
    getindex(DataArray{T<:Number,N},Union(Array{T,1},BitArray{1},Ranges{T})) at /scratch1/tmp/kmsquire/.julia/DataFrames/src/dataarray.jl:362
is ambiguous with 
    getindex(DataArray{T,N},Union(BitArray{1},Array{Bool,1})) at /scratch1/tmp/kmsquire/.julia/DataFrames/src/dataarray.jl:337.
Make sure 
    getindex(DataArray{T<:Number,N},Union(BitArray{1},Array{Bool,1}))
is defined first.
```