Add Key#expires? and fix SubKex#expires

Hey, i made some minor changes to the Key API. Please let me know, what you think about it. 

- Add SubKey#expires?

  Returns true if the subkey has an expiration date, else false

- Fix SubKey#expires

  Returns nil if the key has no expiration date

- Add Key#expires?

  Returns true if the key has an expiration data, else false