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, ...).

README

@@ -0,0 +1,9 @@
+unixtime(1) converts a unix timestamp (number of seconds since epoch) 
+into a human readable date.
+
+INSTALL
+=======
+./configure
+make
+make install
+