This commit enables PWM on pin 11 instead of pin A4 on MKR1000 improving UNO pinout compatibility

Change digitalPinToInterrupt to return pin, and use g_APinDescription[pin].ulExtInt to map pin inside attachInterrupt/detachInterrupt

This brings things inline with the atachInterrupt documentation for
usage:
attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);

Fix #126

The pin mapping is still subject to change, better to not distribute
files that are not final.

The variant's file compliance is now determined using a define named

ARDUINO_SAMD_VARIANT_COMPLIANCE

that refer to the minimum core version required for this variant file
to work.

Pins A1/A2 uses TC4 to generate PWM, the same used in Servo library,
so both cannot work together.

The attribute ATTR_ANALOG is needed to select DAC, so it has been
removed from pins A1..A5 that doesn't have a DAC.

The selection of the correct timer peripheral is now
decided through the PIN_ATTR_{TIMER/TIMER_ALT} attribute.

See #30

Now PA14 is connected to Arduino Zero digital pin 2(NO-PWM) and PA08 is connected to digial pin 4(PWM)
Also connected PA13 to Arduino Zero ATN pin

Bootloader is activated by quickly double tapping to the reset button

Signed-off-by: Thibaut VIARD <thibaut.viard@atmel.com>