Add xslt_stylesheet

[Disinterpreter]

Add ngx_http_xslt_module.
http://nginx.org/en/docs/http/ngx_http_xslt_module.html
It's module needed for https://github.com/arut/nginx-rtmp-module/wiki/Getting-number-of-subscribers

[brocaar]

Feel free to create a pull-request with these changes 😄 This was a research project while I was working on a streaming project which I'm not actively maintaining anymore. However, I'm more than happy to review pull-requests...