Add support for Ruby Time/Date/DateTime objects in #generate_key, etc.

[ribose-jeffreylau]

Currently, only Integer time is supported:

rnp = Rnp.new
k = rnp.generate_key( {
  primary: {
    type: 'RSA',
    length: 4096,
    userid: 'hello test',
    usage: [:sign],
    expiration: (Time.now + 60000).to_i,
  },
})

If a Time/Date/DateTime object is supplied, it would fail:

rnp = Rnp.new
k = rnp.generate_key( {
  primary: {
    type: 'RSA',
    length: 4096,
    userid: 'hello test',
    usage: [:sign],
    expiration: Time.now + 60000,
  },
})
# => Rnp::Error: rnp_generate_key_json failed - (rc: 0x10000002): Bad parameters
#    from /opt/ribose/bundle/gems/rnp-1.0.0/lib/rnp/error.rb:29:in
#    `raise_error'

ruby-rnp should be able to accept a Time/Date/DateTime and convert it
to an Integer when necessary.