[OpenSIPStack] OpalOSSEndPoint change request

[Matthias D. <mdr...@gm...>]

Hello Joegen,

I have a change request. If it's ok please commit the changes. Patch 
files are in attachment.

Motivation: With my changes it's possible to set your own subclassed 
OpalOSSUserAgent and SessionManager. The default  behavior is  the same 
as before.

Matthias