MAINT: Add FutureWarnings for changes of np.insert/np.delete

In numpy 1.8. boolean arrays will be treated like boolean indexing. Also special casing of scalars will be removed from both functions.

@charris wrote some warnings, is this the right way to do those? @certik this adds warnings for changes done in PR gh-452 and would be nice in 1.7. so those can be done in 1.8.

Just to be clear, I removed those scalar special cases, because it just doesn't make any sense to delete/insert from/into a 0-d array if you give an axis that is not `None`, so I really can't see anyone using that, but a warning can't hurt.