Add Items to ScrollList Dynamically
Formalize ItemMetadata to Predict Maximum Item Size

Formalize the concept of item metadata to include the maximum
width and height of the item sizes. This helps positional calculations
when dynamically injecting items into the ScrollList.
Fix Broken Tests
Add New Unit Tests for Dynamically Added Items
Fix Broken Logic in VerticalLayout#getCurrentItemIndex

The VerticalLayout#getCurrentItemIndex method will throw if no
item intersects the viewport center. In such cases, return the index
of the item that is nearest the viewport center.
Merge pull request #27 from WebFilings/add_items_to_scroll_list_dynamically

Add Items to Scroll List Dynamically
Merge pull request #28 from WebFilings/fix_broken_current_item_index

Fix Broken Logic in VerticalLayout#getCurrentItemIndex
Merge branch 'master' of github.com:WebFilings/wf-uicomponents into BOOKS-3560
Add handling of dynamic item insertion to ScrollBar.js and add associated test coverage