Add key navigation

**SUMMARY**
The ability to navigate the ScrollList by pressing the keys that normally are used for navigation in the browser was requested. 

**SOLUTION**
A new class, KeyNavigator, was implemented. It is instantiated from within the ScrollList and scrolls the ScrollList in response to keypresses.

**UNIT TESTS**
A spec for KeyNavigator was added with tests for each of the key presses that it responds to.

**HOW TO +10/QA**
In your wf-uicomponents clone:
``` bash
git fetch && 
git checkout BOOKS-3672_add_key_navigation && 
./init.sh && 
grunt qa
```
The tests should all succeed.
In the ScrollList demo, you should be able to press navigation keys and see the appropriate responses.

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Please review: @timmccall-wf @lancefisher-wf @tonyhelvey-wf