Add new SNMP enumeration modules

Adds modules for 

https://community.rapid7.com/community/metasploit/blog/2014/05/15/r7-2014-01-r7-2014-02-r7-2014-03-disclosures-exposure-of-critical-information-via-snmp-public-community-string

Note that the disclosure blog published a little early. I'll be landing these myself momentarily (while the commit logs make it look like they're mine, they're really [Deral Heiland](https://twitter.com/percent_x)'s).