Incorporate suggestion from Jameson on keeping pointer contents alive

Defined types c_CholmodDense which mirrors the cholmod_dense C struct and a wrapper
type CholmodDense that retains the Julia matrix when constructed from one and retains
the pointer itself when constructed from *cholmod_dense returned from C.

Still need to add a finalizer for the CholmodDense type and create
similar types for CholmodSparse, CholmodFactor and CholmodTriplet.