Added my implementation of unixtime.

This implementation has very few reasonable code (call atol() and ctime()),
but has some nice standard program stuff (parse options, print help, ...).

Makefile

@@ -0,0 +1,5 @@
+help:
+	@echo "Run ./configure first!"
+	@echo "Or even better: take a look at the file README."
+install: help
+all: help