Added an addEdl function

Added:
a function that adds an edl file to a playing video.
stubs in ExternalPlayer, UPnPPlayer, and PAPlayer.
xbmc.player python bindings.

Possible bug: I enabled edls for internet streams. It seems to work most of the time, but seeking throws it off.

C++ isn't my strongest language, so I may have not gotten the pointer/referencing right.