Raspisump

- hcsr04sensor v1.6 module no longer handles rounding of readings. Now handled by Raspi-Sump
- reading.py now using depth method in hcsr04sensor as depth_metric and depth_imperial are deprecated.

- Add section to conf file to change line color in the charts.
[charts]
line_color = FB921D default

- Add hostname to email alert body to better identify which instance of raspisump is alerting
- Add a heartbeat email at a user defined interval to ensure that email is working as intended

- Fix charts displaying improper hour formatting on x axis when using matplotlib version 2 in Raspbian Stretch.

- prevent application from logging negative waterlevels.

- Add error logging to identify possible faulty cables or sensors. Versions
prior to this would only hang and consume 100% CPU. As of this version a
faulty cable or sensor will cause the application to exit gracefully and write
a log to '/home/pi/raspi-sump/logs/error_log' alerting you to a faulty cable or
sensor.

Example;

2016-11-26 10:26:42,\*\*ERROR - Signal not received. Possible cable or sensor problem.