Add on_ssl to data handlers

Data handlers now receive the on_ssl(client_hello) event.

Unlike in connection handlers we do not pay attention to what the
handler returns, this is just to allow handlers to do analysis on the
client hello like looking for thing like bad versions or ciphers.
Add log_attack_event to handlers

Shorthand for creating an attack event and logging it to the event log.
Add more detection of client hello issues

Client hello issues are now detected in data handlers. This moves the
detections of anon ciphers and SSLv2 to the insecurecipherdetection and
weaktlsversiondetection data handlers and adds checks for the NULL cipher and
SSLv3.

This resolves #2
Merge pull request #4 from google/data_on_ssl

Add more cipher/TLS version issue detection