Improved external interrupt handling

• Interrupts can now be enabled calling enableExternalInterrupt(gpio_port, gpio_num, trigger, priority) instead of configuring the EXTI / SYSCFG / NVIC registers manually
• EXTI0-4 interrupts are automatically handled in external_interrupts.cpp, providing EXTIX_IRQHandlerImpl() for the user implementation. There is no more need to call saveContext() / restoreContext() or clear the pending flag, as this is now done automatically. This is done to simplify implementation of the interrupts for the user and to be more consistent with EXTI5-15 interrupts, which were already handled this way.
• Improved readability / documentation of the L3GD20 driver & tests.