rtpjitterbuffer: add "do-deadline" property

Submitted by Miguel París Díaz @mparisdiaz

Link to original bug (#743945)

Description

Created attachment 296048
Patch v1

I have noticed that gstjitterbuffer adds an initial delay of "latency" in ms retaining the firsts packets.
This behaviour can be useful in some cases, but not in others cases where the time of the media establishment is too important.
To solve this, I have added a new property to configure the behaviour of the jitter buffer: "do-deadline".
By default it has the value of TRUE, so the element has the same behaviour that until now.

Patch 296048, "Patch v1":
0001-gstrtpjitterbuffer-add-do-deadline-property.patch

Version: 1.4.5