Fix paste-manager scrollIntoView() behavior

The previous logic assumed that @quill.container was a fixed height.
Fixed by using the viewport-relative offset given by getBoundingClientRect().
Merge pull request #271 from voxmedia/paste-scroll-v2

Fix paste-manager scrollIntoView() behavior
Merge pull request #270 from voxmedia/backspace-first-char

Fix backspace-at-index-0 behavior by ignoring negative start indices
Make the blockquote format exclusive
Always return a DOM node from switchTag()

Previous: `dom(aDiv).switchTag('div') == dom(aDiv)`
Expected: `dom(aDiv).switchTag('div') == aDiv`
Merge remote-tracking branch 'upstream/develop'
Merge branch 'switch-tag-noop'
Merge branch 'blockquote'