RTL support

A few days ago, I've converted a site to rtl using the `bi-app` mixins for sass. Then I discovered bi-app-less and I think it could be a good approach to bring RTL support for Bootstrap3.
Asked on Issue #9913

It requires changes across all framework files, but it doesn't overrides styles and has no extra maintenance. Keeping the development with LTR in mind.

The entire framework is configurable via `@text-direction` variable supporting ltr or rtl

Changes that I can list

* Replace horizontal rules by its mixin version
* Removed bs .border-radius mixin to use instead bi-app mixins
* Added direction to body
* `pre` and `code` maintain `direction: ltr`
* Carousel uses custom mixin for left or right transition
* Popover left/right was left intact
* Bi-app mixin integrated into the mixins.less
* docs.css modified for rtl demo

Javascript seems to work fine, I have not fully tested on all browsers. 
Demo here [http://geedmo.github.io/bootstrap-rtl/](http://geedmo.github.io/bootstrap-rtl/)

May be this is not a valid pull-request but I've created it in order to see the changes and get some feedback. Surely it requires more work 
Bi-app [https://github.com/anasnakawa/bi-app-sass](https://github.com/anasnakawa/bi-app-sass)