Encourage users to install the ruby-full meta-package.

* This will also install ruby-dev, which is needed for compiling any C
  extensions.
Emphasis installing ruby from the package manager.

* Most users just need a Ruby so they can run sass or jekyll.
  Whereas RVM/rbenv are more suited for developers and power-users.
Organize in-page links by installation method than by platform/tool.

* Combined the Package Management Systems sub-list into the top list.
Split the "Third party tools" section into "Installers" and "Managers".
Added ruby-build and ruby-install to the Installers section.
Added chruby to the Managers section.
Put Installers before Managers.
Add a summary to the Installers section.
Link to the chruby section.
Merge branch 'installation_instructions' into master

Conflicts:
	en/installation/index.md

Resolved by incorporating the small addition introduced
by commit 0cdb69467390021aaac323d3d2226d8b8d6b0cab into
the restructured installation page.
Small fixes on installation page (en)
Add some links on installation page (en)
Unify markdown for shortcut reference links
Add section about ruby-build (en)
Some improvements on installation page (en)

* clean up OS X section
* minor update of Gentoo section
* minor modifications and typo fixes