Removes USE_PROVIDED_MAVEN_TGZ flag, and all related code

This support is being removed from the Scala PR validation build.
Updates support for building from custom Scala

Reworks a bit the flags
Adds support for versions.properties file
Adds ability to force the cache functions
Renames the cache functions
Adds a basic configuration to build from custom Scala
Adds some documentation about release, release-dryrun, scala-local-build and nightly
Forces Scala branch to build for pr validation
Merge pull request #21 from skyluc/new-zinc-build/scala-to-scala-ide-build

Updates Scala to Scala ide build