Change get_value to support AbstractArrays

This change allows DeExpr to support other types of arrays. In my case, I'm trying to support DataArrays from the DataFrames package. In limited tests, it works for these as shown in the following example. 

```julia
julia> using DeExpr

julia> using DataFrames

julia> y = DataArray(rand(20)); z = DataArray(rand(20));

julia> @devec qq = z + sum(y)

julia> qq
20-element Float64 Array:
 11.7672
 11.3512
 11.2738
 11.3375
 11.5469
 11.0168
 11.3229
 11.3306
 11.5245
 11.8921
 11.6288
 11.8499
 11.3345
 11.4582
 11.4204
 11.2061
 11.138 
 11.3784
 11.2421
 11.1906

julia> q = DataArray(Float64,20)   # for a DataArray result
20-element Float64 DataArray:
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA
 NA

julia> @devec q[:] = z + sum(y)

julia> q
20-element Float64 DataArray:
 11.7672
 11.3512
 11.2738
 11.3375
 11.5469
 11.0168
 11.3229
 11.3306
 11.5245
 11.8921
 11.6288
 11.8499
 11.3345
 11.4582
 11.4204
 11.2061
 11.138 
 11.3784
 11.2421
 11.1906

julia> z[[1,5]] = NA
NA

julia> @devec q[:] = z + sum(y)

julia> q
20-element Float64 DataArray:
   NA   
 11.3512
 11.2738
 11.3375
   NA   
 11.0168
 11.3229
 11.3306
 11.5245
 11.8921
 11.6288
 11.8499
 11.3345
 11.4582
 11.4204
 11.2061
 11.138 
 11.3784
 11.2421
 11.1906

```