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
Pretty dang sure... check out the screencasts they show what I am describing. All I do in those videos is start dragging in one direction, then it will snap to the page when I change directions of my drag (because of the call to _resetPeekState). Check it out on mobile for yourself, this cannot be the desired behavior. 
Also, from what i get out of the comments it wants to reset the page when the page hits the top or bottom of the viewport. Why? Wouldn't we want the page to always follow the users finger while dragging? Why would we ever want to snap while the user is still dragging?