Added shared tooltips option to show multiple timeSeries values in a graph

Hi.

I've added a new graph option "tooltip" shared to show all values at once in the same time if graph contains more than one time-series. This option enables also a mode "x" crosshair  line to let us to follow the exact time. Works also for stacked and cumulative tooltips.

The Shared Tooltip only works for time-series with equal length else  it will show the original single tooltip.

It is completely backward compatible.

NOTE: text size has been reduced to let us more information with less tooltip area. 

![image](https://cloud.githubusercontent.com/assets/5883405/4371601/c55bc6d0-4319-11e4-8498-3790f09d6d68.png)