Allows requiring individual components

[mmrtnz]

• Fixes require(ing) individual components #363
• Users can now require individual components like so:

var Checkbox = require('material-ui/lib/Checkbox');
var EnhancedButton = require('material-ui/lib/EnhancedButton');

instead of

var {Checkbox, EnhancedButton) = require('material-ui');

• Some components return an index.js and require an additional step, for example:

var MenuComponents = require('material-ui/lib/menu'); // Returns an object pointing to Menu components
var Menu = MenuComponents.Menu;
var MenuItem = MenuComponents.MenuItem;
...
var {Menu, MenuItem} = require('material-ui/lib/menu'); // Alternative approach using destructuring

these include

• Menu
• Styles
• Tabs
• Toolbar
• Utils