Fix #1741: sbt.ExtractAPI: extract annotations

This is necessary for correct incremental recompilation but is also used
by sbt to find tests to run (for junit they should be annotated @org.junit.Test).

I added an sbt scripted test to verify that JUnit now works, to run it:
$ sbt
> scripted discovery/test-discovery

1 files changed, 17 insertions, 0 deletions

diff --git a/sbt-bridge/sbt-test/discovery/test-discovery/project/DottyInjectedPlugin.scala b/sbt-bridge/sbt-test/discovery/test-discovery/project/DottyInjectedPlugin.scala
new file mode 100644
index 000000000..ec22870f4
--- /dev/null
+++ b/sbt-bridge/sbt-test/discovery/test-discovery/project/DottyInjectedPlugin.scala
@@ -0,0 +1,17 @@
+import sbt._
+import Keys._
+
+object DottyInjectedPlugin extends AutoPlugin {
+  override def requires = plugins.JvmPlugin
+  override def trigger = allRequirements
+
+  override val projectSettings = Seq(
+    scalaVersion := "0.1-SNAPSHOT",
+    scalaOrganization := "ch.epfl.lamp",
+    scalacOptions += "-language:Scala2",
+    scalaBinaryVersion  := "2.11",
+    autoScalaLibrary := false,
+    libraryDependencies ++= Seq("org.scala-lang" % "scala-library" % "2.11.5"),
+    scalaCompilerBridgeSource := ("ch.epfl.lamp" % "dotty-sbt-bridge" % "0.1.1-SNAPSHOT" % "component").sources()
+  )
+}