[#538] Edit install from package docs

@amercader @nigelbabu I feel bad because I edited the package install docs pretty heavily, so rather than pushing it to your branch I've made this pull request against your _538-package-install-docs_ branch. Would you mind taking a look at this? And if it doesn't horrify you too much then consider merging it into _538-package-install-docs_ and then we can merge _538-package-install-docs_ into master and release-v2.0

After editing this, I'm left with one burning question: What about people who have a package install of CKAN _not_ on Ubuntu 12.04 64-bit, and want to upgrade? This may include most people who have package installs of CKAN, as prior to 2.0 Ubuntu 10.04 was the recommended OS. The upgrade instructions have nothing for them. It seems to me, that they could uninstall the ckan package, then install CKAN 2.0 from source, then migrate their db and search index?

The other thing that comes to mind, if you read the source install upgrade docs they mention a whole bunch of things that seem like they might also be relevant to package install upgrades and should maybe be added. There's a couple of things that the package install upgrade docs mention that are missing from the source install ones, too. But at this point, I'm starting to lose the will. This is what we get, for having three different upgrade procedures.

Here's the commit message:

This is some fairly heavy editing:

- Some general editing: correcting typos and formatting errors,
  re-writing some sentences in ways that (I hope) makes them clearer

- Use |solr| substitution everywhere (spells Solr consistently with a
  capital S)

- Also use |postgresql| which spells PostgreSQL with the correct caps

- Use |production.ini| substitution for path to production.ini file

- Put the note about Ubuntu 64 only right at the top, and point them to
  the source install

- Also moved _Common error messages_ link to top, where it's more
  prominent (as on source install page)

- Removed stuff about virtualbox, ec2, rackspace, etc. This seems
  unnecessary to me

- Removed "The overall process is the following:" bullet-list from the
  top, it's unnecessary and seems to imply that upgrading is part of the
  installing process which doesn't make any sense

- Numbered the major steps of the install process, like in the source
  install

- Also within major steps, broke things down into individual numbered
  steps to make things clearer and easier to follow

- Tweaked the note about installing solr and postgres on separate servers

- Rewrote the stuff at the top of the _Upgrading_ section, I hope this
  makes it a bit clearer about major vs minor versions and the two
  different upgrade processes

- Moved moving the production.ini file into its own step, so that people
  don't miss it