SparseArray with dtype != float64
It would be nice to support sparse arrays with `dtype` that is not just `float64`.  Specifically, I have some data that is `int8` but sparse, it would be nice to be able to create a `SparseArray` with the `SparseIndex` feature tracking which values are set and which are not set.

Obviously, `np.nan` can't be used to indicate not a value, but maybe a default can be provided (in my case, zero is a nice default).
