restructured DSL overview
added pruneBranches option for Git SCM
Merge pull request #237 from CoreMedia/prune-branches

Added pruneBranches Option for Git SCM
Merge pull request #236 from CoreMedia/overview

Restructured DSL Overview
moved PerforcePasswordEncryptor to javaposse.jobdsl.dsl.helpers.scm package, refactoring, added tests