added: url option to set proxy for http streams

Hi,
this is my first trial for contribute to xbmc code. I don't have build environment and I couldn't build and try this code. It should work I guess :)

I did it for using proxy without enable for hole XBMC. For add-on's mainly.

I need some feedbacks about this feature. Is it necessary or how is the code etc.

TODO: [DONE] Better log message to distinguish from XBMC proxy settings ("Using url option proxy %s")