Add pr-rebuild configuration.
Removes the old script, adds documentation stub
Merge pull request #20 from skyluc/new-zinc-build/clean-up

Removes the old script, adds documentation stub
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
Fixes support for scala-pr-validator

Uses a provided versions.properties file
Keeps defind MAVEN_ARGS
Swaps scala hash and version parameters, because hash is less used.
Added some doc.
Forces Scala branch to build for pr validation
Revert ordering of parameters - need to squashed