Defined a CholmodSparse type

but my external constructor from a SparseMatrix{Tv,Ti} is not working
as hoped.  I'm pushing this commit in case someone can try
```julia
require("suitesparse")
using SuiteSparse
A = sprand(8,4,0.25);
ca = CholmodSparse(A)
```
and tell me why I get a no method error.