feat(expandable): Add 'expandRow' and 'getExpandedRows'

[rafaelb]

Features required for project I am currently working on.

[swalters]

@rafaelb please rebase this PR into one commit. thanks.
http://stackoverflow.com/questions/5189560/squash-my-last-x-commits-together-using-git/5189600#5189600

[rafaelb]

@swalters Done

[JLLeitschuh]

Does this still work for all of the aria roles? Or do those need to be inverted as well?
This seems fine to me otherwise.

[swalters]

@rafaelb To be consistent with our other api methods, can you please change expandRow to toggleRow? This will allow the method to expand or collapse a row.

[rafaelb]

@swalters I think that toggling is already handled by this method: https://github.com/angular-ui/ui-grid/blob/6709846/src/features/expandable/js/expandable.js#L145

For the project we are working on, we needed the ability to expand rows (or not do anything if they are already expanded)
I based this method on the "selectRow" method of the selectable feature:
https://github.com/angular-ui/ui-grid/blob/6709846/src/features/selection/js/selection.js#L177

What I think the best solution would be is to have "expandRow", "collapseRow" and "toggleRow" methods.
What do you think?
If you agree, I will add a "collapseRow" method.

[swalters]

@rafaelb You suggestion is good.

[rafaelb]

@swalters Added 'collapseRow' method