updated building instruction to reflect 2.10 being the default version

Did a full run of version mentions in the doc. I had missed some 2.9->2.10 changes
Complements and finishes https://github.com/scala-ide/scala-ide/pull/282

Voluntarily kept occurences of explicit passing of `-Pscala-2.10.x` in late command lines
(e.g. memory leak test) since those are still valid & in dev doc, not using default shouldn't
phase anyone.
Merge pull request #38 from huitseeker/feat/update-building

updated building instruction to reflect 2.10 being the default version
Adds the doc for the Mark Occurrences preference

Creates a new page in the Helium section, for part of the Hydrogen documentation
which is updated.
Added a link to some Tycho tutorial
Tweaks the mark occurences documentation
Merge pull request #42 from skyluc/issue/mark-occurrences-preference-1001466

Adds the doc for the Mark Occurrences preference
Adds doc for library classpath validator
Merge pull request #43 from skyluc/issue/classpath-validator-preference-1001482

Adds doc for library classpath validator