Added square tabs option, like Soda

Soda has an option that lets you go back to the "classic" style tabs, meaning ones with rounding. Some of us like the square look, so I added "flatland_square_tabs" as an option users can toggle to get "square" style tabs

![Screen Shot 2013-04-14 at 5 22 09 PM](https://f.cloud.github.com/assets/168193/378740/8029b2ec-a55a-11e2-9ce5-9775b5a5fd8c.png)