Rewrite the basic installation instructions for recent changes

The main point of this change is, now that the PHP backend
is gone, to make the basic instructions focus on getting
the Django backend to work.  It gets you running with the
Django local development server, and then suggests options
for webservers and WSGI application servers to use.

There are various other small changes that I've made here:

* Add a note that you can create the PostgreSQL database
  however you like, for people who know how to do so.  (For
  example, I don't see an advantage in using password-based
  over ident-based authentication.

* Remove the literal list of Python packages and their
  versions since they will be instantly out-of-date.

* Remove links to generic help pages (the Ubuntu PostgreSQL
  page and a German language page of help on Apache
  configuration) - I think these are likely to be less
  helpful for people than just having them use their
  own initiative.

* Put back some instructions for installing CATMAID at the
  root of a virtual host with Apache.

* Remove all references to PHP or PHP packages

* Remove the Django installation page (since that's now
  basic installation)

* Add a placeholder page about installing from the
  CATMAID AMI on EC2, and list it as an alternative
  up front.

* Add a placeholder section about Nginx + Gunicorn

* Recommend the use of virtualenvwrapper instead of
  a bare virtualenv, which will be much easier for
  most people to use.

* Add a link to the bug about the NumPy dependency problem.

* Various bits of rewriting to (hopefully) make the
  instructions clearer and some corrections to
  reStructuredText syntax.