Initial cut at client-level server API

Server---Build-level-API.md

@@ -0,0 +1,31 @@
+The server aspects to sbt build will be exposed to build/plugin authors as follows:
+
+
+1.  A new key available which can be used to interact with the server:
+
+```scala
+val uiContext = taskKey[sbt.UIContext]("All user interaction should be driven through this interface.")
+```
+
+2. A new interface:
+
+```scala
+package sbt
+trait UIContext {
+
+  /** Prompts the user for input, optionally with a mask for characters. */
+  def readLine(prompt: String, mask: Option[Char] = None): Option[String]
+
+  /** Sends an event out to all registered event listeners. */
+  def sendEvent[T: Format](event: T): Unit
+}
+```
+
+3. An addition to plugin API:
+```scala
+trait Plugin {
+  def serializers: Seq[(AttributeKey[_], Format[_])] = Nil
+}
+```
+
+# TODO - More information about the changes...