Add MPD.Processing.groupRecords
Decouple connection and authentication

MPD.Handle.open no longer attempts to authenticate
connection automatically.
Authenticate via MPD.Interact.authenticate.
Add examples/hmpc: a simple command-line driver
Add MPD.Processing.foldRecords
Merge branch 'decouple-auth'
Merge branch 'hmpc'
Merge branch 'groupRecords'
Merge branch 'foldRecords'