Add suggestion for secure key generation

Take advantage of the key derivation now performed to use a simple 64-byte random hex string for the key.  This should be easy to store in a database, or however else it needs to be.
Finishing sentences helps

No, really
Merge branch 'patch-1' of git://github.com/namelessjon/libmacaroons

* 'patch-1' of git://github.com/namelessjon/libmacaroons:
  Finishing sentences helps
  Add suggestion for secure key generation
make check for previous merge