fixed index lookup to handle non unique names

With a CSV with non unique column names, the `make_unique` method is used to generate unique column names internally. But it was not possible to lookup using a generated column name as the `lookup` dict in `Index` was not getting updated.

With a CSV like:
````
A,B,C,C,C,D,E,F
0,1,2,3,4,5,6,7
1,2,3,4,5,6,7,8
2,3,4,5,6,7,8,9
````

Before:
````
julia> df = readtable("t.csv")
3x8 DataFrame:
        A B C C_1 C_2 D E F
[1,]    0 1 2   3   4 5 6 7
[2,]    1 2 3   4   5 6 7 8
[3,]    2 3 4   5   6 7 8 9

julia> df["C_1"]
ERROR: key not found: "C_1"
 in getindex at dict.jl:502
 in getindex at /Users/tan/.julia/DataFrames/src/dataframe.jl:388
````

With this change:
````
julia> df = readtable("t.csv");

julia> df["C_1"]
3-element DataArray{Int64,1}:
 3
 4
 5
````