'Install Groovy' section added
Introduction added
Page structure added
Restructure page. The proposal to change.
Title "IDE plugin" changed. Typo fix.
"Groovy 2.1" section filled.
"Groovy 2.2" section filled.
Page "core-getting-started.adoc" finished (Some links are require clarification).
".groovy" removed in shell expression
Links to version-scheme.adoc added
Links on invokedynamic-support.html added. Section "Legacy Groovy 1.0" updated
Page modified for a single version.
Merge pull request #221 from bura/doc-core-getting-started-new-struct

GROOVY-6268: some more getting started details