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.