nrf51822/log: Decrease the UART interrupt priority

Now the priority is lower than the RADIO and TIMER0.
log: Add several logging functions
log: Change log_print() API to log_printf()
examples/logging: Update example to use new functions
timer: Rename timer_cb to timer_cb_t
timer: Move cb parameter from timer_create() to timer_start()
nrf51822/timer: Fix timing drift in repeated timers
examples: Add radio-connection-slave example