Use axisLabelColour (if set) for html labels, or omit inline css color

HTML labels are currently hardcoded to use `this.opts.color`, which is used by flot for colouring gridlines. Since this is set as inline css, it's impossible to override with a stylesheet (using eg `.axisLabels {color: black;}`) and effectively means that the axis labels have to be the same colour as the gridlines.

This should probably use `this.opts.axisLabelColour` if it's been set, and otherwise omit the inline `color` declaration so that some global style can be applied.

Specifically the line
```javascript
'color: ' + this.opts.color + '; ' + 
```
should be replaced with something like
```javascript
(this.opts.axisLabelColour ? 'color: ' + this.opts.axisLabelColour + '; ' : '') +
```