Add Button atom to xpub-ui

src/atoms/Button.js

+import React from 'react'
+
+const Button = ({ children, type = 'button', disabled}) => (
+  <button
+    type={type}
+    disabled={disabled}>{children}</button>
+)
+
+export default Button

src/atoms/Button.md

+A button.
+
+```js
+<Button>Save</Button>
+```
+
+A button can be disabled.
+
+```js
+<Button disabled>Save</Button>
+```

src/index.js

+export { default as Button } from './atoms/Button'
 export { default as Checkbox } from './atoms/Checkbox'
+export { default as Icon } from './atoms/Icon'
 export { default as Menu } from './atoms/Menu'
 export { default as Radio } from './atoms/Radio'
 export { default as Tags } from './atoms/Tags'