If the user sets the _sbt.boot.credentials_ system property or the
_SBT_CREDENTIALS_ environment variable, then that file gets parsed for a
realm, hostname, username and password which is added to the
CredentialsStore static instance.
The sbt.classifiers property sets the classifiers for sbt and
scala.classifiers sets it for Scala. Note that when creating a
project, Scala and sbt are retrieved right after project setup.
Changes to *.classifiers will only apply if project/boot is
removed or a new version of Scala is retrieved.
Mark Harrah
Aaron D. Valade