ipykernel messes with signal handlers

This is a follow-up of #49. That PR added support for restoring a custom `SIGINT` handler. This was used by the SageMath kernel to restore its custom `SIGINT` handler. However, this really should be made more general.

Given that upstream Python does not support restoring arbitrary signal handlers (see https://bugs.python.org/issue13285 for that), I would like to add optional support for [cysignals.pysignals](http://cysignals.readthedocs.io/en/latest/pysignals.html) in the IPython kernel.