libmsip API update

[Erik Eliasson]

_libmsip API updates_

The API to the libraries are often not very well defined.
For libmsip we have distributed all header files, even those
that are only used internally in libmsip.

SipStack will be split into two classes, one that is the main
access point to the library, and one that is only used internally
(SipStack and SipStackInternal).

Header files for the classes that are implementing the SIP stack
logic, and should not be accessed from outside the the library
will not be installed/distributed. I leave them in the
"include/libmsip" folder for now, but they should perhaps be in
a different location.

--Erik

-- 
Erik Eliasson <eliasson at it.kth.se>