Add javascript injection exploit for firefox 15-22

Been sitting on this for almost a year now, but it required the user to bring up the Web Console prompt manually, so was user-assisted and meh. Today I thought of a way to get this exploit working without the user-assisted bit! So here is an exploit that will get you shells on any version of firefox on any OS between versions 15 and 22.

#### Verification

- [x] Download a version of firefox 15-22, any OS is fine
- [x] Configure your payload and run the module

        msf> use exploit/multi/browser/firefox_tostring_injection
        msf> set LHOST 0.0.0.0

- [x] Browse to the displayed URL, get that shell
- [x] Run autopwn, you should get a shell as well