installation: improved instructions and checks

• Completed the list of pre-requisites, e.g. numpy.

• For Debian, updated apt-get install instructions, adapted Apache vhost snippet generation, added instructions about creating SSL certificate and about activating vhosts.

• For Red Hat, updated Apache vhost snippet generation to include explicit IP address of the box and to specify the WSGI socket directive. (Both often needed on various RH based systems.)

• Updated installation commands with respect to running under www-data user identity. Clarified wording in various places.

• Added test for mistakenly-nested `invenio' symlink creation mishap during installation of the Python module.