src/common.h: Added common.h

common.h contains some miscelleneous preprocessor macros.
src: Added support for button inputs

This feature allows us to watch for falling edge on defined GPIO pins and report
this to the MQTT broker. The GPIO lines are software debounced and have the
internall pullups activated on startup.
src: Convert simple for loops to use the new times() macro
src: Disable IP packet reassembly

We do not need it and it saves around 300 bytes of RAM.
src: Moved subscribe calls to main.c
src/main.c: Added binary outputs

Pins declared as output set state to high/low on receiving MQTT message with
"true"/"false" as content on their respective topic.
src/main.c: Changed structure of sensor handling functions

This commit splits most of the sensor handling code to separate functions. This
makes the #ifdef-ed parts of the main function slightly more readable.