Sparse ellipsis slicing

This add indexing with `Ellipsis` (`...`) to sparse matrices. This closes [issue #2721](https://github.com/scipy/scipy/issues/2721). 

There is a possible bug with NumPy's indexing with `Ellipsis` in its matrices which I choose not to replicate. I made an issue [here](https://github.com/numpy/numpy/issues/3646).