Matt's MPTCP code

Matt's MPTCP code for ns-3.

This is for reviewing; the patch doesn't apply to ns-3-dev.  If you want to run the code, checkout the 'outsourcing_CC' branch from github here:  https://github.com/mkheirkhah/mptcp 

See also these web links for more information:
https://www.nsnam.org/wiki/GSOC2015MpTcpImplementation (wiki)

http://codereview.appspot.com/248320043 (Morteza Kheirkhah's development branch)

[Vedran Miletić, 2015-07-13 21:43:31]

Matt and I discussed on IRC how to proceed with this, given the large amount of
code.

Generally, I like the approach. As discussed on IRC, "MPTCP is a shim layer to
aggregate TCP subflows", and Matt's code is designed around this idea (e.g.
MpTcpSocketBase is a subclass of TcpSocketBase), and existing applications
continue to work.

Matt is already working on rebasing this over Natale's refactoring. Once this is
done I suggest the code review made in form of many patches, where each
implements a single feature or so and provides tests and documentation of this
particular feature.

Finally, there is leftover commented code. This is neat to have when developing,
but is not very helpful when reviewing code.