Add new multiappsrc or dynappsrc element with multiple output streams

Submitted by Justin Kim @joykim

Link to original bug (#725187)

Description

if Audio/Video streams are separately given, playbin isn't able to handle two sources.
This is a suggestion about how to deploy multiple appsrcs and to use with playbin.

The basic idea started from here; http://lists.freedesktop.org/archives/gstreamer-devel/2014-January/045771.html

Simple documentation is included in codes.