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>

.gitignore

@@ -9,8 +9,6 @@
 *.html
 .deps
 .libs
-examples/joystick
-examples/signal
 compile
 config.*
 configure
@@ -26,7 +24,6 @@ aclocal.m4
 ar-lib
 autom4te.cache
 bench
-test
 libuev*.tar.*
 libuev.pc
 !libuev.pc.in