Added Medias code and documentation

Fix for Issue #1711

Added media.less and edited bootstrap.less and responsive.less to
manage media components.

Added a section to the documentation (Media, in components.html) outlining how Medias work. Edited bootstrap.css and bootstrap-responsive.css to add the code for the demos

Signed-off-by: Giuliano Velli <wpbrains@giusi.org>

less/bootstrap.less

@@ -52,6 +52,7 @@
 
 // Components: Misc
 @import "thumbnails.less";
+@import "media.less";
 @import "labels.less";
 @import "badges.less";
 @import "progress-bars.less";