.gitignore: Added .map to .gitignore

.map is a file generated by the build process
src: Added sensors_send()

The sensors_send (and the sensors_send_* macros) contain common code which was
previously duplicated in the main function.
src/main.c: Moved weather parsing/displaying logic to new function

Previously, the parsing and displaying was done directly in the mqtt message
handler. This commit moves this part of the code to the newly created
display_weather function.
src/main.c: Remove extraneous call to display_therm()

For some reason, the display_therm function was called every time the
kalive_timer expired.
src: Make the temperature and humidity sensors optional

By defining HAS_DHT and/or HAS_DS in the config.h file, the firmware can be now
compiled without support for any of these sensors. This also stops posting the
sensor information to the corresponding MQTT topic.