remove superfluous bytes when reading messages

Merge branch 'fix-listento-panic' into 'master'

Bugfix: fix panic in ListenTo for some msg types

Closes #55

See merge request gomidi/midi!8

add Bytes method to SMF

fix the smfwriter logger

fix the smfwriter logger

add JSON marshaling/unmarshaling methods to SMF type

improving rpn/nrpn

add Messages

improve rpn_nrpn situation

further rpn/nrpn improvement