Allow atvalue indexing in categorical axes

[iamed2]

julia> using AxisArrays

julia> using AxisArrays: CategoricalVector

julia> A = AxisArray(zeros(3,3), Axis{:a1}(CategoricalVector([1,55,9])), Axis{:a2}([11,22,33]))
2-dimensional AxisArray{Float64,2,...} with axes:
    :a1, [1, 55, 9]
    :a2, [11, 22, 33]
And data, a 3×3 Array{Float64,2}:
 0.0  0.0  0.0
 0.0  0.0  0.0
 0.0  0.0  0.0

Previously:

julia> A[atvalue(55), atvalue(22)]
ERROR: ArgumentError: index Value(55, tol=0) not found
Stacktrace:
 [1] axisindexes(::Type{AxisArrays.Categorical}, ::AxisArrays.CategoricalVector{Int64,Array{Int64,1}}, ::AxisArrays.Value{Int64}) at /Users/ericdavies/.julia/v0.6/AxisArrays/src/indexing.jl:249
 [2] getindex(::AxisArrays.AxisArray{Float64,2,Array{Float64,2},Tuple{AxisArrays.Axis{:a1,AxisArrays.CategoricalVector{Int64,Array{Int64,1}}},AxisArrays.Axis{:a2,Array{Int64,1}}}}, ::AxisArrays.Value{Int64}, ::AxisArrays.Value{Int64}) at /Users/ericdavies/.julia/v0.6/AxisArrays/src/indexing.jl:98

Now:

julia> A[atvalue(55), atvalue(22)]
0.0

[ajkeller34]

LGTM, but I will wait a day to merge in case someone else has comments.