example: Add PIR example

This patch adds the initial files to implement a PIR-based motion
detector application. When motion is detected, the application should
inform through advertising data using GAP Broadcast role. This patch
doesn't Bluetooth Advertising integration yet, it implements the sensor
data reading only.
README: Add brief instructions file
example: Add Temperature sensor example

This patch adds a temperature broadcaster application. The temperature
is read from a temperature sensor at a interval of one second. If the
value has changed since the last read, the application should inform
the new temperature (and humidity) through advertising data using GAP
Broadcast role.
build: Allow examples to compile unused platform files

This is needed, for example, to use buttons in nRF1822 platform.
build: Add specific build files in .gitignore

Before, only the "build" path was excluded in .gitignore. But if the
user replaces BUILD_PATH, the files built in compilation will not be
ignored. This patch adds the generated common files and removes the
default build directory.
radio: Add documentation about radio events
ll: Add missing const modifier in ll_advertise_start()
ll: Check for errors in ll_advertise_start()
ll: Check for errors in ll_advertise_stop()
ll: Check for errors in ll_init()
ll: Improve callback handling

This patch modifies the code to only one of the callbacks be
responsible to call radio_send().
nrf51822: Add references for the interrupt levels
nrf51822/timer: Add some documentation and references
nrf51822/timer: Better error handling
nrf51822/timer: Increase default number of timers

Some examples need more timers to work properly.
nrf51822/radio: Update reference
nrf51822/radio: Document radio_set_tx_power()
nrf51822/radio: Document radio_init()
nrf51822/radio: Document ch2freq()
nrf51822/radio: Document radio_send()