D-Link hedwig.cgi - Cookie bof - return to system

This is the second exploit from the advisory by Roberto Paleari (http://www.exploit-db.com/exploits/27283/). As in the module https://github.com/rapid7/metasploit-framework/pull/3385 I have created a ROP to system and this starts the telnetd.

![dlink_hedwig_cgi_bof](https://cloud.githubusercontent.com/assets/497520/3132822/8bd5fca2-e813-11e3-8fd3-a081bf87f8b2.png)

This vulnerability is not only in the DIR-645. It is also in the firmware of the DIR300 and DIR600. I think there could be other devices out there that are also vulnerable.
@m-1-k-3 do you want to give a chance to the modification in this pull request? I'll work on these embedded devices pull requests starting probably tomorrow! And starting with this pull request. Others pull request using EchoCmdStager can be rebased later, once this one is landed.

I'll try to put some testing environment ready unless you say you weren't able to emulate with qemu! (I see references to qemu on the module so I guess you were able to emulate).

Thanks!