Indexing by an array of values

[ajkeller34]

Now that #52 has been merged, this PR attempts to address #84, except for the "by default" part. Simply broadcast atvalue:

julia> A = AxisArray(rand(7), Axis{:x}(-3:3))
1-dimensional AxisArray{Float64,1,...} with axes:
    :x, -3:3
And data, a 7-element Array{Float64,1}:
 0.44085 
 0.77351 
 0.192314
 0.904178
 0.912603
 0.485005
 0.730789

julia> A[atvalue.([-2,1])]
1-dimensional AxisArray{Float64,1,...} with axes:
    :x, [-2, 1]
And data, a 2-element Array{Float64,1}:
 0.77351 
 0.912603

With many dimensions, I concede that typing atvalue repeatedly might become tiresome. Perhaps in some follow-up PR I might implement a macro (@value?) for splicing atvalue repeatedly into an expression if a user wants to index all dimensions by value. I would guess that should be possible though I haven't tried or thought too deeply about it.

[ajkeller34]

Hmm, I guess this doesn't work on 0.5. The failures are related to the broadcasting changes in 0.6, so this is blocked until 0.5 support is dropped for AxisArrays.

[timholy]

I have no reservations about dropping 0.5 now, though perhaps we should finish & merge #90 (and then tag) first?

[ajkeller34]

I'm in no hurry to merge this. I'm not sure what is left to do in #90 (seems like requiring 0.6 is the simplest solution?) but let me know if I can help somehow.

[timholy]

#102 has been merged, so the way is clear here.