Go back to using TCP sockets for communication

The new loadtest.js script indicates that for whatever reason, using
TCP connectivity to R worker processes is far more stable than
Unix domain sockets. This commit switches us back to TCP but also
abstracts out a transport layer that makes it trivially easy to
switch back and forth between the two, or add new transport types.
Remove socket dir from scripts and docs
Merge pull request #44 from jcheng5/tcp-transport

Go back to using TCP sockets for communication