Update CONTRIBUTING.md

Spelling fix - removed unnecessary "by"

# Before 

```
# Contributing to Famo.us

Active involvement from the community is essential to help make famous the most
capable and performant front-end JavaScript framework out there. You can help by
reporting bugs, fixing bugs, adding features, contributing new modules and by
by providing feedback.
```

# After
```
# Contributing to Famo.us

Active involvement from the community is essential to help make famous the most
capable and performant front-end JavaScript framework out there. You can help by
reporting bugs, fixing bugs, adding features, contributing new modules and by
providing feedback.
```
Update CONTRIBUTING.md

Fixed broken link 

# Before
```
If you'd like to contribute a fix for a bug you've encountered, first read up on
[how to report a bug][reporting-bugs-and-other-issues] and report it so we are
aware of the issue.
```
# After
```
If you'd like to contribute a fix for a bug you've encountered, first read up on
[how to report a bug](#reporting-bugs-and-other-issues) and report it so we are
aware of the issue.
```
[DOC] CSS classes are set on target div
[TWEAK] Use enum form this._springState
Support function-wrapped transforms in Scene definitions.

Is that the right terminology ("function-wrapped")?

I was trying to do this:

```javascript
    // create the main context
    var mainContext = Engine.createContext();

    // set the perspective (what type of lense?)
    mainContext.setPerspective(1200);

    var initialTime = Date.now();

    // define the main scene.
    var mainScene = new Scene({
        id: "root",
        target: [

            { id: 'background' },

            {
                id: "centerSpin",
                origin: [0.5, 0.5],
                transform: function() {
                    return Transform.rotateY(.001 * (Date.now() - initialTime));
                },
                target: {id: 'logo',}
            }

        ],
    });

    var background = new Surface({
        properties: { backgroundColor: '#25475E'},
    });

        var logo = new ImageSurface({
            size: [400, 400],
            content: '/content/images/logo.png',
            classes: ['backfaceVisibility']
        });

    mainScene.id['logo'].add(logo);
    mainScene.id['background'].add(background);

    mainContext.add(mainScene);
```

but the animation wasn't working. Adding those lines to Scene.js fixed it.
May fixed issue #175
[Fix] Enable chaining for transitionable.halt()
Implement toggleClass
remove diff from readme
Add analytics to readme
fix size dirty checking

`_sizeDirty` was always false, and there was a typo when checking if there is a size
Add getSize to ScrollContainer
verify existence of performance.now
Remove References to submodules from README
Update Contributing to no longer reference submodules
Update emails in travis.yml
Fix linter to fail on eslint

Originally we were not explicitly running one linter after another, and if eslint failed it would not trigger a failure on travis.
reference window.performance.now rather than global performance
sizeDirty flag removed
change DEFAULT_OPTION direction to Y