RFC: Make sparse multiplication work for AbstractArrays

This gives warnings of the type:
```
Warning: New definition 
    *(SparseMatrixCSC{T1,Ti<:Integer},AbstractArray{T2,1}) at linalg/sparse.jl:13
is ambiguous with 
    *(AbstractArray{T,N},BitArray{N}) at linalg/bitarray.jl:67.
Make sure 
    *(SparseMatrixCSC{T,Ti<:Integer},BitArray{1})
is defined first.
```
but works fine. What is the better solution to get rid of the warnings?