Use 0-based index for switching toolbars

* `switchToolbar` should expect a zero-based index of button to select
* `onKeyDown` should call `switchToolbar` with an index
Demo - simplify the demo start process with an npm start script
0.1.0
Feature - programmatically modify toolbar contents

* setContent - clear and set the toolbar items
* addContent - append a toolbar item to the end (right-most side)
* emptyContent - remove all items from the toolbar
Break out keytable to `lib/keytable.js`
BugFix - id of `0` is falsey

derp!
Merge remote-tracking branch 'remotes/kumavis/patch-1' https://github.com/maxogden/toolbar/pull/2
Merge remote-tracking branch 'remotes/kumavis/patch-4' https://github.com/maxogden/toolbar/pull/4

Conflicts:
	package.json
Merge remote-tracking branch 'remotes/kumavis/patch-5' https://github.com/maxogden/toolbar/pull/5

Conflicts:
	index.js
	readme.md
Merge remote-tracking branch 'remotes/kumavis/patch-3' https://github.com/maxogden/toolbar/pull/6
Replace keytable.js with 'vkey' module