Remove fake elements after they are used
fix(uiGridHeader): Fix header height calculation

When the header cells were long enough that they wrapped inside the header
container, they were causing the header height to be calcuated as double
what it should have been.

This has been fixed by making the header cells floating, and defaulting
their width to 0. Their width is calculated dynamically within
uiGridStyle.
Updating TODO
Minor syntax cleanup
Adding commented out code that will make the scrollbar slide in when the mouse hovers the grid
Merge branch '3.0' of https://github.com/angular-ui/ng-grid into 3.0