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
Whitespace and the first lines in the comments for the new observables are full sentences. Please put a period at the end.