Merge child-processes-endpoint-4696-5

Authors: ashfall, therve
Reviewers: exarkun, tom.prince
Fixes: #4696

Add a ProcessEndpoint facility to easily connect a protocol to a process.


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@37840 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb