timer: Extract timer_create() from timer_start()

This patch breaks timer_start() API in two other APIs: timer_start()
itself and timer_create(). This was made because it is common in BLE
to have the same timer triggering with different timeout and data.
examples: Update timing example with new timer APIs
nrf51822: Change log interrupt level to HIGHEST

This patch changes the log interrupt level to allow put DBG statements
inside others interrupt handlers.
radio: Change radio_init() return type
nrf51822: Add radio.c to Makefile.platform
radio: Add Radio APIs
nrf51822: Implement radio_init()
nrf51822: Implement radio_register_handler()
nrf51822: Implement radio_recv()
nrf51822: Implement radio_stop()
examples: Add radio-observer example

This patch adds an example that implements the GAP observer role using
radio and timer APIs directly.
nrf51822: Implement radio_send()
examples: Add radio-broadcaster example

This patch adds an example that implements the GAP broadcaster role
using radio and timer APIs directly.