Improvements to Morris.Hover

List of features:
* Introduction of ```cssPrefix``` option that is used with ```class``` (if class name starts with ```-```),
* Absolute positions (that's it, overflowing the chart container) with ability to fix it to left, right, top, or bottom from passed parameters ```x```, ```y```,
* Exposure of ```hoverOptions``` to Line, Area, and Bar charts (example usage: ```days.html```),
* Tag class attribute builder using ```cssPrefix``` and ```class``` (now passed as array of classes).

Harmless to current ```master```. This only adds some new functionality which I find useful and probably others will too.