Tooltip: add a styleClass attribute

[VictorRos]

I'm submitting a enhancement

[x] bug report => Search github for a similar issue or PR before submitting
[x] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[x] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35

Current behavior
Cannot personalize multiple tooltips by overloading CSS classes.

Expected behavior
Adding an attribute like [styleClass] on tooltip element to obtain the ability to have differents tooltips.

What is the motivation / use case for changing the behavior?
In our project, we have 2 buttons with tooltip on it but we cannot customize each other independently by overloading the .ui-tooltip class.

This is not a vital enhancement but it would be appreciated to have this feature in the near future.