Support area chart to behave like line chart with semi-transparent fill

#199:

* added ```behaveLikeLine``` option (defaults to ```false```),
* added ```fillOpacity``` option (accepts ```0.0-1.0``` or ```auto``` which means ```0.8``` for line-like behaviour and ```1.0``` for cumulative behaviour; defaults to ```auto```),
* refactored ```Morris.Line::drawSeries``` to separate lines and points drawing for each series (was very useful in implementing line-like behaviour),
* added **three** specs to ```Morris.Area``` to test the new functionality,
* replaced area fill spec to not rely on the hsl value but check if the fill value is not equal to stroke color.

**Passed Travis CI and is ready to be merged. Does not break current functionality and is 100% backward compatible.**