XBMC RetroArch integration and some cores

This adds some additional retroarch cores as well as a script generation package to be used in conjunction with XBMC and the AdvancedLauncher addon, making it possible to use XBMC as an emulator frontend.
Are you sure? Compare:
```
xbmc-plugin.advanced-launcher    xbmc-plugin-advanced-launcher-2.5.7
```
with
```
xbmcPlugins.advanced-launcher    xbmc-plugin-advanced-launcher-2.5.7
```
The former is more coherent, derivation name and attribute name are more similar. Also didn't convention change to use dashes rather than camelcase for package attributes just to make drv<->attr names more coherent?