create_addon: adds support for multiple extension points in addon.xml

This PR adds a possibility to use multiple extension points in addon.xml. it uses PKG_ADDON_EXTENSIONS variable in package.mk. The format of the data in variable is as follows: 
extension_point:python_script  
More extensions are separated by a space. In case the extension point is xbmc.service third parameter is allowed - when the service should be started - startup | login. In this case e.g.:
xbmc.service:python_script:startup