Added rules to remove rounded borders if input is prepended or appended

Just added two rules to the stylesheet so the rounded border is removed when an appended or prepended input extension is added.

Haven't updated the Less rules.

http://twitter.github.io/bootstrap/base-css.html#forms

![switch-example](https://f.cloud.github.com/assets/1098227/513548/1a19c834-be41-11e2-973c-2969aa599057.jpg)