CollectDCrypto: Reload authfile when modified

This requires the 'watchdog' module. A general purpose single
file monitor has been put in a new file 'helpers.py' since it
didn't really fit anywhere else.
Move temp_file() to tests/t.py
Add test for bucky.helpers.FileMonitor
CollectDCrypto: Add tests for authfile load/reload
Small change in FileMonitor and test for py3
Merge branch 'collectd_crypto.reload_authfile' into tmp