Update TabbedBar.js

Occasionally the total tab/button width exceeds that of the bar width, forcing the last tab to a new line, out of the tab's viewport.

If a bar width is specified, do not use a horizontal tab layout, instead set the `left` property of each tab to ensure all tabs always remain in the bar's viewport.
Update TabbedBar.js

Wrong conditional for `horizontal` layout
Merge pull request #8 from allenhartwig/patch-3

Update TabbedBar.js