Option to scroll by total pixels scrolled or proportionally

Browser-sync should synchronize scroll by **total pixels scrolled** too (which was the behavior of this feature before #31). This PR fixes #64.

`scrollProportionally` option was added to browser-sync options. It's default is `true`. [Wiki/options](https://github.com/shakyShane/browser-sync/wiki/Options) should be updated after this PR get merged.

Simple changes to `browser-sync-client.js` were needed. Just conditionals to use raw or proportional scroll values.