PPA for mednafen, xbmc (PL6) and Stella

Tested with RetroRig 0.9.5b, upstream from 14/08/07
OS: Ubuntu 14.04, scratch install after apt-get update/upgrade

This pull request addresses:

#74 "PPA: discussion and progress tracker" : can be closed

#72 "splash screen" : changes applied to xbmc
test with xbmc in retroRig mode (Nouveau driver): check 
test with xbmc in native mode (Nouveau driver): empty, file /usr/share/xbmc/media/Splash.png exists
test with xbmc in retroRig mode (Nvidia driver): check 
test with xbmc in native mode (Nvidia driver): check

#68 "xbmc doesn't fullscreen after game exit"
test on primary display (Nouveau driver): check 
test on secondary display (Nouveau driver): only ok, if xbmc not fullscreened. No Unity top bar in mednafen fullscreen mode 
test on primary display (Nvidia driver): check 
test on secondary display (Nvidia driver): ok, but top bar shows up when mednafen fullscreens 

ESC, Save, reload: check (#77)

No sound issues with mednafen + Nouveau due to SDL driver (#48)

Different icons for xbmc in native and RetroRig mode: check

rescan: not OK. Missing file /usr/share/applications/gp_autodetect_xbmc.sh (not scope of this pull request)
(fixed with copying from RetroRig/XBMC-cfgs/extra and changing permission)

PS3 controller hotplugging: ok

Output of '~/.retrorig$ find . -type f -exec grep -H "/home/test" {} \;': http://slexy.org/view/s21Qv7uWKF (#77) precompiled python scripts and .mess affected

Recap: Transition from libregeek to PPA is OK. XBMC and mednafen have issues in fullscreen mode beeing placed under Unity's top bar. The Nvidia and Nouveau OpenGL driver for X11 show different functional behaviour and graphic issues. Nouveau doesn't display the native xbmc logo, but fullscreening for both medanfen and xbmc works on the primary display. Nvidia's driver supports fullscreening on the second display, but mednafen and sometimes xbmc get placed under the Unity top bar. 

[EDIT]: This pull request now also addressed #78 "Stella fails to launch with OpenGL video settings and Nvidia drivers". Scripts to build Stella have been added to this pull request. Stella was uploaded to the PPA. The installation process for RetroRig now installs Stella via apt-get.