btn-xxx-inverse class for hight contrasted website

I have edit buttons.less to add inverse feature on button (like navbar).

In hight contrasted WebSite (navbar, header) standards btn-xxx styles can be useless in site header or body.

btn-outline getbootstrap.com homepage inspire this feature.

I use the variable @body-bg but I think it's not the best way (a transparent variable will better but need to add new variable).

#Exemple or Test
```
Options : http://jsfiddle.net/hQv22/
Sizes : http://jsfiddle.net/A5V3R/1/
Active state : http://jsfiddle.net/mvrq4/2/
Disabled state : http://jsfiddle.net/mvrq4/3/
Button tags : http://jsfiddle.net/mvrq4/4/
Glyphicon : http://jsfiddle.net/mvrq4/5/
DropDown : http://jsfiddle.net/mvrq4/6/
Button groups : http://jsfiddle.net/mvrq4/7/
Button toolbar : http://jsfiddle.net/mvrq4/8/
Button toolbar Sizing : http://jsfiddle.net/mvrq4/9/
Button toolbar Nesting : http://jsfiddle.net/mvrq4/10/
Button toolbar Vertical variation : http://jsfiddle.net/mvrq4/11/
Justified button groups : http://jsfiddle.net/mvrq4/12/
Split button dropdowns : http://jsfiddle.net/mvrq4/13/
Dropup variation : http://jsfiddle.net/mvrq4/14/
Button addons : http://jsfiddle.net/mvrq4/15/
Inverted navbar : http://jsfiddle.net/mvrq4/16/
With badge : http://jsfiddle.net/mvrq4/17/
```