Add UMFPACK

[pv]

Now that UMFPACK is not any more used in scipy.sparse.linalg, only pysparse has easy-to-use UMFPACK wrappers. scikits.umfpack has only a very low-level interface.

So it would be useful to add UMFPACK to scikits.sparse, together with the other SuiteSparse items.

[njsmith]

I don't have time to work on this, but there's a partially written UMFPACK wrapper in the 'umfpack' branch that might be a good start if anyone else wants to take a shot at it: a264635

SPQR would also be pretty trivial to add, since it uses the same data structures as CHOLMOD.