0.3.1

Added

• command line parameter --db-min and --db-max to set the dB range of the
  spectrum and waterfall plot
• ui config options db_min and db_max to set the dB range of the spectrum
  and waterfall plot
• display current azimuth and elevation of the current observations vessel
• display rotator position if rotctld address is set
• command line parameter --rotctld-address to enable rotator monitoring
• config option rotctld_address to enable rotator monitoring