Recipies for working with Unix time format

Add a recipe for getting a Date from a Unix time.
Add a recipe for getting a Unix time from a Date.
Also added section title to the current date for cross reference.
Merge branch 'master' into unix-time

Conflicts:
	primitive-data/dates/current-date/current-date.asciidoc
Merge pull request #144 from stevenproctor/unix-time

Add two recipes for Dates to/from Unix time