Added social sharing provider : addthis

I have added the configuration for addthis social sharing. I have modified the`_config.yml` with the addthis hash. I have also created a folder called sharing-providers under `_includes`.

I don't see a custom folder under includes to add my custom sharing which I worked on [here](https://github.com/plusjade/jekyll-bootstrap/pull/15). 

Shall I go ahead and add it ?