cron: New API for absolute timers

This patch adds support for at/cron-like jobs that need an absolute
timer.  The API supports waking up with an UEV_HUP callback event if the
wall clock changed due to NTP or user input while waiting.

- uev_cron_init()
- uev_cron_set()
- uev_cron_start()
- uev_cron_stop()

A very simple cronrun example/test program is also included.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

examples/.gitignore

@@ -0,0 +1,4 @@
+joystick
+signal
+cronrun
+test