Add timestamp property type to the Consonant specification

Add a definition of the timestamp property type to section 3 of the
specification, with an example showing how it could be used.
Make amendments to timestamp property definition

Remove trailing whitespace in some paragraphs and reword the values
section.
Merge branch 'adamcoldrick/timestamp-property' of https://github.com/CodethinkLabs/consonant into adamcoldrick/timestamp-property

Conflicts:
	consonant.mdwn