New interrupt handling framework which covers both sleeping and NOT sleeping nodes  (#142, #149, #166, #154)

* Decreased the number of items of the _sensors array #117
* Fixed gateway crashing on node check-in

* Fix compilation and other issues

-) Enums (in particular the sensor types) start with 0, so we have to allow a sensor type of 0
-) Add missing SensorMQ::setTargetGas method
-) Make gas type identifiers for MQ sensors const static to preserve 6 bytes of memory per MQ instance

* Fix wrong array index (typo)

* Optimize the MQ sensors by using one static array for the default curve coefficients. This adds three extra pointers, but saves 48 bytes for each additional MQ sensor after the first

This reverts commit 34e073fc.

* Added functions to get sleep mode from NM

* Implemented Timer class

* Commented the Timer class

* Improved Timer class

* Replaced battery report with Timer

* Using Timer in SensorRainGauge class

* Add custom report time to Sensor

* Used Timer for force update

* Added safeguard to SensorDigitalOutput

* Added capability to turn on with a timer for SensorDigitalOutput

* Tested all the new functions and fixed the issues

* Stopping the timer if setInputIsElapsed is on and a LOW value is set

Add debug message with encryption status

* Added new sensors, updated docs

* Added SensorRain

* Added SensorSoilMoisture

* Updated docs with how to contribute instructions