Support direct use of syscall() function (for #13).

It seems that nodejs has a backend that uses syscall()
directly instead of accept or accept4. This is a bit annoying,
but we can handle stubbing out this case.
Adding epoll support (fix #13).

When the dummy socket is swapped in, we need to potentially
remove references from epoll descriptors for the original socket.
We already have dup, dup2, etc. that will track the state of
different descriptors, since this is pretty straight-forward.
Merge pull request #14 from amscanne/nodejs_support

Node.js support.