Linear indexing into block arrays

This issue relates to #168, where linear indices in a BlockIndex led to a stack overflow.

A question that arises is whether linear indexing should be supported and if so, how. Currently, there is this behaviour:

julia> using BlockArrays
A = [1 2; 3 4]
julia> A = [1 2; 3 4]
2×2 Array{Int64,2}:
 1  2
 3  4

julia> A[Block(1,1)]
2×2 Array{Int64,2}:
 1  2
 3  4

julia> A[Block(1)]
2-element Array{Int64,1}:
 1
 3

julia> A[Block(2)]
ERROR: BlockBoundsError: attempt to access 2-element Base.OneTo{Int64} at block index [2]

Indexing a matrix with Block(1) yields the first column only, but Block(2) does not yield the second column. The blocksize of the matrix is 1 indicating that A[Block(1)] == A may be more consistent. But that would be an incompatible change.

Start by reviewing issue #168 and the current BlockIndex behavior shown in the examples. Compare the semantics of A[Block(1)] and A[Block(2)] with the matrix blocksize, then determine whether linear indexing should be supported and whether compatibility constrains the choice. Done means the intended indexing behavior and its compatibility implications are resolved.