Expose processTweens

For #109: https://jeremyckahn.github.io/shifty/doc/shifty.html#.processTweens

Also:

• Switches the test framework to Jest
• Drops browser test runner (consider using https://www.npmjs.com/package/ndb if you need an interactive debugger)
• Updates all of the devDependencies to their latest versions

Fixes null reference error

Fixes #105.

Performance improvements

2.5.0 switches to batched tween processing (#87), which significantly improves performance for situations when there are many tweens running at once. You can see a performance comparison between Shifty, GSAP, and jQuery here.

2.5.0 also fixes a minor bug that was introduced in 2.4.0.