Exploit mixin cmdstagerbourne

This pull request adds the new Exploit Mixin CmdStagerBourne and an example sshexec module to demonstrate its features.  It works in a very similar fashion to the Windows CmdStagerVBS Mixin in that it Base64 encodes the payload, uploads it by echoing it to a file, decodes it and ultimately executes it.

It is dubbed the Bourne stager but the with the sshexec module, it has also been used with bash and zshell.

The sshexec module has been tested on Fedora 64-bit, Ubuntu 32-bit, and OSX 10.7 and the four available DECODER values are functioning on all tested systems.