Integrate David's ssl-enum-ciphers improvements

From this thread: http://seclists.org/nmap-dev/2014/q1/105

* Extensions now better supported in tls.lua
* ssl-enum-ciphers sends all EC options to ensure servers reply with
  supported EC suites
* tls.lua supports multiple messages of a single type within 1 record
* tls.record_buffer will read an entire TLS record into a buffer
* ssl-date and tls-nextprotoneg updated to use tls.record_buffer