Modify runtime to be an average value of all files

Hi Guillaume,

this is not a PR to be merged immediately. Please have a look at it first and tell me what you think. Until now the runtime was decided by the last scanned episode. If this is a "special" episode with longer or shorter runtime than the other episodes, runtime will be set to this longer or shorter runtime. Not ideal IMHO.

With this PR I changed this. An average value will be calculated and put in the episode runtime field. Episodes with 0 set as runtime in nfo will be ignored in the calculation. 

Michael