Make JUnit tests runnable from the command line

- Update the build to SBT 0.13.0
 - Update Scala dependency to 2.11.0-M6
 - Add dependency on JUnit
 - Update DottyTest to use the setting `useJavaCP = true`
 - Update PathResolver to be aware of the protocol for an embedded Scala
   instance to discover the boot and app classpaths from the host SBT
   classloader

After this, I see two test failures:

    [error] Test test.ShowClassTests.loadClassWithPrivateInnerAndSubSelf failed: java.lang.Error: null
    [error]     at dotty.tools.dotc.core.Symbols$class.newStubSymbol(Symbols.scala:188)
    [error]     ...
    [info] Test test.ShowClassTests.loadDotty started
    [info] Test test.ShowClassTests.showReflectAliases started
    [error] Test test.ShowClassTests.showReflectAliases failed: java.lang.Error: null
    [error]     at dotty.tools.dotc.core.Symbols$class.newStubSymbol(Symbols.scala:188)