UIComponents changes for HYDRA-706

### What I Did
Added events for onScrollToItem so when the peek interceptor tells a page to snap we can disable zoom canvas until the snap animation is done.
I also removed a check in the peek interceptor that caused the page to automatically snap when a direction changed happens while dragging

### How to test
The events can be tested along with HYDRA-706, however one can test the direction change issue by doing the following
clone wf-uicomponents and checkout this branch
cd pdf_client_services/viewer/bower_components
rm -rf wf-uicomponents
ln -s ~/path/to/clone/of/wf-uicomponents
rm -rf wf-js-viewer
ln -s ~/path/to/clone/of/wf-js-viewer
cd pdf_client_services
node run path/to/doc.pdf
npm start

Assure you can drag up and down all during the same gesture (without picking up your finger) without the page snapping

Screencasts:
Before:
http://www.screencast.com/t/H1554gpPJF
After:
http://www.screencast.com/t/3g784aiG
The page will follow the user's fingers, for sure. I think the thing you are missing is that the "peek" behavior is a combination of panning the individual items and the list as well. You can pan an item if it has overflow in the direction of your drag, when you hit the boundary of the item's viewport, the drag event ends up translating the list map, so that previous/next items are scrolled into view.