removed color treatment of outline and Webkit styling of outline for easier implementation of Twitter Bootstrap

* The default outline treatment of outline is ```invert``` which ensures that outline colour  will always have sufficient contrast from the background it is on.
* ```.tab-focus()``` mixin was styling the background colour to ```#333``` , this poses a bit of a restriction for implementers of twitter bootstrap since if the background is darker than white, there may not be enough contrast to see the outline, causing an accessibility issue.
* Additionally removed the webkit outline style and left it to the default recommendation ```outline: thin dotted``` to ensure best cross-browser consistency [http://a11yproject.com/posts/never-remove-css-outlines/]

Great work again TB team, looking forward to v3.0.3

/cc @mdo @cvrebert