Add keyboard accessibility enhancement

[B-Reif]

This PR adds keyboard accessibility to react-slider (and closes #41).

refactors

• _move() now takes a new slider value (used to take a mouse position)
• _move() logic for calculating value and position diff are moved into new functions
  • _getValueFromPosition(position) returns a corresponding slider value
  • _getDiffPosition(position) compares the argument to state.startPosition and inverts if needed
• _onMouseMove() and _onTouchMove() now use these new functions and pass a value to _move()

additions

• add _moveUpOneStep() and _moveDownOneStep() functions
• add tabIndex and aria-* attributes to handle
• add _onKeyDown(), _onBlur(), _getKeyDownEventMap(), and _createOnKeyDown()
  • _createOnKeyDown() is called on focus
  • _onBlur() is called on focusout

misc

• don't blur the activeElement if it's the currently active handle's ref

[mpowaga]

Thanks for working on this @B-Reif! Is this supposed to work with example in index.html or it needs to be updated? Because I can't change the slider value with arrow keys using the example. Tested on OS X with Safari and Chrome.

[B-Reif]

I just tested it with Chrome without any issue. Are you able to tab-focus the slider handles, and then the keybinds aren't working? Or can you not even tab to them?

[mpowaga]

Just checked again and it worked this time after setting a focus with tab.

It looks like Y-axis is reverted, i.e. up arrow moves the slider down and down arrow up. Is this intentional? I'd expect that it worked the other way around.

[B-Reif]

I was going by examples from http://oaa-accessibility.org/example/32/ where Up/Left always decrements and Down/Right always increments.

The vertical slider example has invert: true so the arrow key functionality seems backward; it works as expected if it's not inverted.

We can flip the arrow key functions if invert: true. This might cause some confusion for blind users though. Let me know which way you'd like to go.

[mpowaga]

We definitely should stick to accessibility standards. Looks good to me. Can you just add missing semicolons? Then I'll merge. Thanks!

[B-Reif]

Ok, should be good to go.