Add Key#expires? and fix SubKex#expires

- 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
Merge pull request #51 from ushis/master

Add Key#expires? and fix SubKey#expires