Add modules for more NTP amplification attacks -- R7-2014-12

As described in:

https://community.rapid7.com/community/metasploit/blog/2014/08/25/r7-2014-12-more-amplification-vulnerabilities-in-ntp-allow-even-more-drdos-attacks

The changes here can be summarized as:

* Add a new mixin for detecting/proving traffic amplification flaws (with unit tests)
* Add modules covering 5 of the 6 vulnerabilities described above
* Update ntp_monlist to utilize UDPScanner and various NTP/etc improvements, aligning the auxiliary/scanner/ntp/ modules more

Validation:

In general, any system that is vulnerable to CVE-2013-5211 (MON_GETLIST_1) is likely also vulnerable to all of these, however to the best of my knowledge only NTP.org variants are actually vulnerable.  I have tested this against NTP versions as old as 4.2.2 and as new as 4.2.7p465 released 08/23/2014, and in all instances I tested properly secured systems (disable querying, disable mode 7) as well as improperly secured systems.