Revert removal of socket_auth.*

This reverts part of commit c767c9589b2f5e6be3d4eac5d9e5c2e51417b984.  Now that
ESP is likely going to be using Remit again, the socket_auth script is once
more useful.
Unignore .gitignore

In general, .gitignore's should be committed and the same across checkouts of a
git repo.  Checkout-specific ignores should be installed in .git/info/exclude.
Clarify the "connection issue" bit.
Explain the socket_auth subdirectory
Make socket_auth.py remove the socket if present

You can't bind to an AF_UNIX socket if the path already exists. Historically,
we've run socket_auth.py inside of a bash loop that removes the socket and then
starts it. However, in order to run inside of supervisord, it's convenient to
instead have the script itself delete the old socket.
socket_auth.py: Work better with unbuffered stdout

In converting socket_auth.py to run under supervisord, we changed to log to
syslog via supervisord. Thus, we want each line of output to show up in a
single write(), so it shows up as a single syslog entry. Unfortunately,
Python's default buffering is too aggresive and we never see any output, so I
switched to unbuffered Python output. That means that print should be given a
single string (built using % escapes) rather than several strings, since each
string will be outputted separately.