add libdiscid as (direct) backend

We now depend on pyton-discid >= 0.5.0
That is where ISRC support was added and the API changed.
remove "lesser" backends, re #48

Some backends have features that libdiscid doesn't have.
That is raw reads and CD-TEXT isrcs.

Having discisrc as a backend is redundant, but this might come in handy
for testing purposes. Having different versions of libdiscid and
discisrc.
display MCN if available