Minor fixes and improvements.

This version introduces the following changes:

- On Ctrl-C and Ctrl-\, instead of raising a signal, return the new
  error code ErrKeyboardInterrupt.

- Add a new function ReadWithTimings which allows to collect the
  timings of all keypresses during password input.  This is intended
  to be used to gather entropy for a random number generator.

- Various cosmetic fixes, mostly in the documentation.