Add missing items to setup.py packages list and MANIFEST.in
Put some extra protection in get_extra_info calls

When shuttdown down a session or connection, calls to get_extra_info could
sometimes fail. This commit puts an extra guard in to protect against that.
Update setup script to allow install without dependencies

This commit updates the setup.py script to allow asyncssh to be
installed even when its cryptographic dependencies are missing. It will
still report an error when you try to import asyncsshif neither PyCrypto
nor PyCA is installed, though.
Update setup.py to use the correct name for the pycrypto dependency