Local timeout that kills sslyze scans -- but doesn't clean up processes

[konklone]

This is a partial fix for #138, that adds a local timeout library that successfully cuts off sslyze processes that get zombified for unknown reasons.

The full solution is to migrate our use of sslyze to the Python API, now that sslyze works on Python 3 and has such an API. This would also help us track down the source of the hangup in a more granular way, as opposed to the process being a black box.

We'd likely still also benefit from this work on a local timeout, and it could be useful for other scanners, so this seems worth doing.

[konklone]

To make this more explicit: if sslyze processes get zombied, I believe they may end up needing a manual kill sweep to clean them up, but the scan can continue. That's why this is only partial.

[konklone]

Closed in favor of #151.