Make bin/openpgp more useful

[devrandom]

I've added code to make it dump packets instead of doing nothing.

Regarding

#!/usr/bin/env ruby -rubygems

unfortunately, on some systems (e.g. Ubuntu Lucid), #! only splits the first two arguments, so the script fails to run.

Regarding decode_count - it's called here in a way which requires it to be public:

  s2k.count = s2k.decode_count(input.read_byte)

(ruby 1.8.7)