demo(tab) display the delete button when hover on the tab instead of the...

demo(tab) display the delete button when hover on the tab instead of the label of the tab, and make the delete button look better

https://github.com/angular/material/issues/62, the 4th demo on tab page when mouse over the lable of the tab, it will show the delete button, but it's really not good for 2 reason.
1. when mouse over the gap between close button and lable, it will flash
2. the button break the layout of the tab.

so I make the button shown when mouse over the tab instead of the lable, and make the delete button looks little better, and when hover on the delete button, display a nice rounded yellow circle.