Centralize strings in prep of private-repo move.

[adriaanm]

All repo url strings are now attributes. Will also need to change some occurrences over in scala/scala:

README.md:100:> set resolvers += "pr" at "http://private-repo.typesafe.com/typesafe/scala-pr-validation-snapshots/"
build.xml:474:    <property name="sbt.interface.url"     value="http://private-repo.typesafe.com/typesafe/ivy-releases/org.scala-sbt/interface/${sbt.latest.version}/jars/interface.jar"/>
build.xml:476:    <property name="sbt.interface.src.url" value="http://private-repo.typesafe.com/typesafe/ivy-releases/org.scala-sbt/compiler-interface/${sbt.latest.version}/jars/compiler-interface-src.jar"/>
scripts/jobs/integrate/bootstrap:21:# - ~/.credentials-private-repo for private-repo.typesafe.com, as follows:
scripts/jobs/integrate/bootstrap:23:    # host=private-repo.typesafe.com
scripts/jobs/integrate/bootstrap:112:privateRepo="http://private-repo.typesafe.com/typesafe/scala-release-temp/"
scripts/repositories-scala-release:3:  private-repo: http://private-repo.typesafe.com/typesafe/scala-release-temp/
scripts/repositories-scala-release:4:  typesafe-ivy-releases: http://repo.typesafe.com/typesafe/ivy-releases/, [organization]/[module]/[revision]/[type]s/[artifact](-[classifier]).[ext], bootOnly
tools/binary-repo-lib.sh:5:remote_urlget="http://repo.typesafe.com/typesafe/scala-sha-bootstrap/org/scala-lang/bootstrap"
tools/binary-repo-lib.sh:6:remote_urlpush="http://private-repo.typesafe.com/typesafe/scala-sha-bootstrap/org/scala-lang/bootstrap"

[retronym]

LGTM!