Add convenience constructors

[dlfivefifty]

I think these should work:

julia> BlockArray{Float64}(1:10,1:10)
ERROR: MethodError: no method matching BlockArrays.BlockArray{Float64,N,R} where R<:AbstractArray{Float64,N} where N(::UnitRange{Int64}, ::UnitRange{Int64})

julia> PseudoBlockArray{Float64}(1:10,1:10)
ERROR: MethodError: no method matching BlockArrays.PseudoBlockArray{Float64,N,R} where R<:AbstractArray{Float64,N} where N(::UnitRange{Int64}, ::UnitRange{Int64})

In the first case, I suppose it should allocate the blocks.