On-Balance Volume (OBV) technical indicator

[pawelfus]

Implement OBV technical indicator:

• - algorithm
• - algorithm tests
• - create a demo (e.g. https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/stock/indicators/apo/)
• - describe params in API (e.g. https://api.highcharts.com/highstock/series.ad.params)
• - add it to list of indicators (https://www.highcharts.com/docs/stock/technical-indicator-series) - if necessary, add to the bottom list too: https://www.highcharts.com/docs/stock/technical-indicator-series#multiple-series-bindings
• - add it to indicators-all.js (http://code.highcharts.com/stock/indicators/indicators-all.src.js)
• - test it in StockTools (e.g. separate yAxis for Oscillators)
• - test it in Popup (e.g. if params are not missing)
• - if necessary, add lang in API (https://api.highcharts.com/highstock/lang.navigation.popup)
• - add the indicator to the demo with all indicators (http://utils.highcharts.local/samples/#view/stock/demo/all-indicators)