use `ping-pong.spotify.nodestuff.net` web service for sendPong

Closes #98.

Squashed commit of the following:

commit 547cb2b1d6394d060d4eda7f58a582bee2bb7882
Author: Denys Vitali <denys@denv.it>
Date:   Thu Dec 11 18:15:25 2014 +0100

    Removed request dependency

commit 435b266c8278885ce42306b46dd2017e3e2ea5b3
Author: Denys Vitali <denys@denv.it>
Date:   Thu Dec 11 18:15:02 2014 +0100

    Replaced request with superagent

commit e661d43a5c59b3757aaa56cd2d0715097c88c516
Author: Denys Vitali <denys@denv.it>
Date:   Thu Dec 11 12:28:44 2014 +0100

    Update package.json

commit e971f192b3082a22da2f4b9ba2bb1199fb460dd8
Author: Denys Vitali <denys@denv.it>
Date:   Thu Dec 11 12:01:08 2014 +0100

    Added request module to deal with sendPong

commit 200f5c0ad151b12de185fc0f8ed6a8b69d00b406
Author: Denys Vitali <denys@denv.it>
Date:   Thu Dec 11 12:00:30 2014 +0100

    Updated Spotify sendPong

commit b08ed0dc4e202644c989fbcf8a9091569230b739
Author: Denys Vitali <denys@denv.it>
Date:   Thu Dec 11 00:42:39 2014 +0100

    Updated Spotify sendPong

    Thanks to @1994rstefan

    https://github.com/TooTallNate/node-spotify-web/issues/87#issuecomment-50890401
spotify: update comment to explain the web service