[RFC/WIP] Few additional methods

[alyst]

Some methods that I introduced, while rebasing JuliaStats/DataFrames.jl#850

• isnull(A::NullableArray) to get the boolean array of A nulls
• unsafe_isnull(A::NullableArray, i::Int...) to check for nulls without checking the validity of the index
• AbstractNullableArray{T, N} typealias of AbstractArray{Nullable{T}, N} (however AbstractNullableArray (without params) is not considered by Julia more specific than AbstractArray, I don't know if it's a known feature/bug)
• isless{T,S}(a::Nullable{T}, b::S) to compare nullables vs nonnullables (nonnullable is always less)