Improve NTP monlist auxiliary module

I made ​​some changes to make the module more similar to a "NTP monlist" scanner.

- add entries counter in the results;
- add RETRY options;
- add SHOW_LIST options;
- add references;
- modify description;

Notes:

	With auxiliary/scanner/ntp/ntp_monlist, auxiliary/scanner/chargen/chargen_probe and
	auxiliary/scanner/dns/dns_amp (https://github.com/rapid7/metasploit-framework/pull/2929/commits)
	we cover the main network services used for nowadays amplification attacks.
	
Demo using ntp pkg (1:4.2.6.p3+dfsg-1ubuntu3.1) on Ubuntu 12.04.4 LTS:

	msf auxiliary(ntp_monlist) > show options

	Module options (auxiliary/scanner/ntp/ntp_monlist):

	   Name       Current Setting  Required  Description
	   ----       ---------------  --------  -----------
	   BATCHSIZE  256              yes       The number of hosts to probe in each set
	   CHOST                       no        The local client address
	   RETRY      1                no        Number of tries to query the alleged NTP server
	   RHOSTS     1.2.3.4    	   yes       The target address range or CIDR identifier
	   RPORT      123              yes       The target port
	   SHOW_LIST  false            no        Show the recents clients list
	   THREADS    1                yes       The number of concurrent threads

	msf auxiliary(ntp_monlist) > set VERBOSE true
	VERBOSE => true
	msf auxiliary(ntp_monlist) > set SHOW_LIST true
	SHOW_LIST => true
	msf auxiliary(ntp_monlist) > run

	[*] Sending probes to 1.2.3.4->1.2.3.4 (1 hosts)
	[*] 1.2.3.4:123 1.1.1.1:123 (lst: 95 sec., cnt: 295]
	[*] 1.2.3.4:123 2.2.2.2:123 (lst: 63811 sec., cnt: 8]
	[*] 1.2.3.4:123 3.3.4.3:21399 (lst: 153575 sec., cnt: 8]
	...
	...
	[+] 1.2.3.4:123 NTP 'monlist' request permitted (480 entries)
	[*] Scanned 1 of 1 hosts (100% complete)
	[*] Auxiliary module execution completed