Minor refactoring of delay variables

This is a preparation for handling multiple interrupts in delay().
It also reduces the size of the interrupt-related variables and
arguments to int8_t as that should be ample enough for any Arduino
implementation.
Added constructor to sleep() for monitoring two interrupts

The new constructor allows two separate interrupts with two
separate trigger policies to wake up device.
Updated BinarySwitchSensorInt to handle two sensors

The new sleep() constructor allows for two interrupt sources
to wake device from sleep. This example is now updated to
illustrate this.
Merge pull request #31 from fallberg/development

Development