Require PyCrypto, remove optional import

PyCrypto is required for its AES implementation that is used
in CollectDCrypto for decrypting encrypted packets.
Add watchdog module to requirements.txt

It is used by bucky.helpers.FileMonitor to monitor a file for
modifications. FileMonitor is used by collectd.CollectDCrypto
to reload the authfile when it changes without restarting
bucky.
CollectDCrypto: Update README.rst