Add support for XhYmZs, in any order, all elements optional

Signed-off-by: Pedro Melo <melo@simplicidade.org>
Improve fail_duration():

 * when passing, make sure trailing \n is removed from message;
 * when failing, make sure the spec is mentioned.

Signed-off-by: Pedro Melo <melo@simplicidade.org>
Prepare release 0.005

Signed-off-by: Pedro Melo <melo@simplicidade.org>
v0.005

        * Add support for XhYmZs expression types