Fixes for ActionBarSherlock on Gingerbread and older

I had to make these fixes in order for the pull to refresh view to display correctly on Gingerbread and older.
``getActionBarBackground`` and ``getActionBarSize`` were returning null and 0, respective, which was causing the view to look completely broken. The progress bar also was showing as a round indeterminate progress bar, so I had to change the parent style.
Also had to depend on actionbarsherlock for this fix, not sure if it is okay or not for this library, but sending the PR for review just in case :)

BTW great work on this library, we replaced Android-PullToRefresh with this one, looks better now! Thanks!