Added DNS and ETag covert channels

DNS and ETag covert channels imlementation was added to BeEF. 

Extensions:
 * etag
 * s2c_dns_tunnel

Modules:
* etag_client
* s2c_dns_tunnel
* test_get_variable

Developers:
* Oleg Broslavsky(@ovbroslavsky)
* Nikita Oleksov(@neoleksov)
* Nikolay Tkachenko (@afr1ka)
* Denis Kolegov (@dnkolegov)

Links:
* [ D. Kolegov, O. Broslavsky, N.Oleksov. Covert Timing Channels using HTTP Cache Headers] (http://www.slideshare.net/dnkolegov/09092014-1)
* [K.Born. Browser-Based Covert Data Exfiltration.] (http://arxiv.org/ftp/arxiv/papers/1004/1004.4357.pdf)
* [W. Alkorn,C. Frichot, M.Orru. The Browser Hacker's Handbook] (http://eu.wiley.com/WileyCDA/WileyTitle/productCd-1118662091.html)