Walkthrough fixes (rebased onto develop)



This is the same as gh-301 but rebased onto develop.

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This PR fixes and updates the Linux/OS X installation walkthroughs using the feedback provided by @ctrueden (see http://lists.openmicroscopy.org.uk/pipermail/ome-devel/2013-March/002377.html)

Main changes:
- Set OpenJDK 7  as the recommended installation package
- Align walkthroughs structures for maintenance ease