Add package lists for Ubuntu precise and Debian squeeze

The config/packages file cannot be used as a list of packages
to give to apt-get install - its format is designed to be used
as build-dependencies of a fake Debian package.  These
distribution- and release-specific new files, on the other
hand, provide a list of packages that should satisfy the
config/packages requirements on Ubuntu precise or Debian
squeeze.  The generic site install file will use files like
these on the matching distribution and release.
Add an example nginx.conf

This example nginx.conf file may be helpful for running
Alaveteli behind nginx - we have tested this with Thin
as the upstream server.
Add the Thin web server to the Gemfile

Thin is a reasonable choice for a production server for a Rails
app behind nginx, which is the configuration that the Alaveteli
install script will use.  Thin is also the preferred server for
local development in Rails 3.2 and later.
Add a sysvinit script for starting Thin

This is a sample sysvinit script to start Thin in development mode.
Normalize whitespace in rails-post-deploy

The rails-post-deploy script unfortunately uses a mixtures of spaces
and 0x09 to represent indentation, so it's not even clear what local
convention to follow.  It appears from the mixture that the intention
is that the tabs should represent 4 spaces, so this commit replaces
them, strips trailing whitespace and changes some non-standard
indentation at the end of a bash heredoc.
Deal with ../logs existing, and log not being a symlink

The rails-post-deploy script would error if the log directory existed
(rather than being a symlink to another directory, for example) and
the ../logs directory also existed.  This commit changes this
behaviour to move the existing log directory out of the way in this
case.

In addition, this commit switches from removing the old symlink and
creating a new one (which creates a gap in time during which the log
directory doesn't exist) to using "ln -snf" to just overwrite any
existing symlink or file.  (Note that this is still not an atomic
operation, but it's bound to leave less time between removal and
creation.)
Add a config option for whether to use mailcatcher in development

Sometimes it's useful to be able to really send mail in development
mode, so add a config option for that.
Exit quietly from purge-varnish-debian if varish is not installed
Add install scripts for Alaveteli

site-specific-install.sh will be called by our generic site
install script in commonlib/bin/install-site.sh

These scripts assume that you have a new installation of
Debian squeeze or Ubuntu precise and then will set up:

 - Alaveteli running in development mode with the Thin web
   server behing nginx

 - The cron jobs that are required for the site to work.

 - A basic Postfix configuration for sending and receiving
   mail.

We also will use this script for generating new AMIs
(Amazon Machin Images) for Alaveteli.

The general.yml configuration file will be created if it
doesn't exist, but if there is an existing copy it won't be
overwritten, so it should be safe to customize that file and
then re-run the install script.