Adding variables to subgrid scope

The PR has code changes for:
1. Grid option 'expandableRowScope' can now be used to add variables to scope of expanded grid.

Now we have 3 options for expanded grid, I wanted to group them into single grid option object, but I am afraid that subgrid feature is already in use at many places and this change might break code.

Also, might be being able to map 'expandableRowScope' to a scope variable of controller will be more useful, but that would have additional overhead of watchers. Please suggest.

2. Column moving feature has issues on IE:
    1. Moved columns are not positioned correctly.
    2. Styles of moving column is not visible.
The PR will fix issue 1 above.