Little improvement on auxiliary/gather/dns_reverse_lookup.rb 

Hi, 

I made this because I had needed it improvement on this module. Now it could be throttled, Could output to a CSV file and all PTR records are maintained in the DB, previousely we only have the last one. 

Be kind, it's my first push, and i'm not a rubyiste.

New options :

```
msf auxiliary(dns_reverse_lookup) > set RANGE 192.168.1.0/24
RANGE => 192.168.1.0/24
msf auxiliary(dns_reverse_lookup) > set OUT_FILE /tmp/rangerdns.csv
OUT_FILE => /tmp/rangerdns.csv
msf auxiliary(dns_reverse_lookup) > set THROTTLE 15
THROTTLE => 15
msf auxiliary(dns_reverse_lookup) > run
[*] Running reverse lookup against IP range 192.168.1.0/24
[*] Throttle set to 15 queries per seconds
[*] Scan result saved in /tmp/rangerdns.csv
[*] #Host Name: newhost.meuiutn.com., IP Address: 192.168.1.145
[*] #Host Name: newhost.miproutn.com., IP Address: 192.168.1.145
[*] #Host Name: newhost.miiiiiiiproutn.com., IP Address: 192.168.1.145
[*] #Host Name: newhost.miautn.com., IP Address: 192.168.1.145
[*] Auxiliary module execution completed


msf auxiliary(dns_reverse_lookup) > notes 192.168.1.145
[*] Time: 2014-04-15 15:36:40 UTC Note: host=192.168.1.145 type=RDNS_Record data=["newhost.miautn.com.", "newhost.meuiutn.com.", "newhost.miproutn.com.", "newhost.miiiiiiiproutn.com."]

[*] exec: cat /tmp/rangerdns.csv

; IP, Host
192.168.1.145,newhost.meuiutn.com.
192.168.1.145,newhost.miproutn.com.
192.168.1.145,newhost.miiiiiiiproutn.com.
192.168.1.145,newhost.miautn.com.
```