Set buffer properties of gstreamer pipeline

Manually set the `playbin2` buffer properties to their default values. The
default values are not documented but hard-coded in gstreamer [1].

This is a starting point for making these properties configurable. For example
we might want higher values on system with unsteady network connection.

[1]: http://cgit.freedesktop.org/gstreamer/gst-plugins-base/tree/gst/playback/gsturidecodebin.c?h=0.10#n1714