Update linux install to clean up outdated and incorrect information

- Don't enable additional repositories (everything is now in main)
  Debian doesn't need contrib or non-free; Ubuntu doesn't need
  restricted or multiverse (for the packages required in this
  guide).
- Install the JRE rather than the JDK, and default to installation of
  OpenJDK7.  Mention how to get Oracle Java, but remove the
  instructions for oab-java6 for security reasons (defaults to
  java6 and won't be kept updated).  Leave a link for people to find
  it, but don't recommend it.
- Default to Ice 3.4 and PostgreSQL 9.1, the default versions in
  the distributions being used for the walkthrough.
- Correct creation of ~/apps/OMERO so it works from any initial
  directory.
- Environment variable setting only needed if not using distribution-
  provided packages.
- Include list of places setting environment variables is possible.
- The list of exports to add to set environment variables was largely
  incorrect.  Replace with a list of environment variables including
  an explanation of what each is for and under what circumstances it
  should be set, and what it should be set to.
Update PostgreSQL requirements

Remove obsolete OMERO versions from the compatibility matrix, and
replace with current PostgreSQL and OMERO releases.  Link to server
backup for dumping the database prior to upgrading.
Update Java installation requirements
Update python installation requirements

Change "should" to "must".
Update Ice installation requirements
Update environment variables needed for installation

Merge the linux-specific section into the main section, removing
all the incorrect parts.
Move client documentation from server installation page