Introduce incremental compiler options.

Introduce a way to configure incremental compiler itself instead
of underlying Java/Scala compiler.

Specific list of changes in this commit:
  * Add a method to `xsbti.compile.Setup` that returns incremental
    compiler options as a `java.util.Map<String, String>`. We considered
    statis interface instead of a `Map` but based on mailing
    list feedback we decided that it's not the best way to go because
    static interface is hard to evolve it by adding new options.
  * Since passing `java.util.Map<String, String>` not very convenient
    we convert it immediately to `sbt.inc.IncOptions`
  * Add options argument to various methods/classes that implement
    incremental compilation so in the end options reach
    `sbt.inc.IncOptions` object
  * Add `incOptions` task that allows users to configure incremental
    compiler options in their build files. Default implementation of
    that tasks returns just `IncOptions.DEFAULT`
  * Both system property `xsbt.inc.debug` and `IncOptions.relationsDebug`
    trigger debugging of relations now. In the near future, we should
    deprecate use of `xsbt.inc.debug`.

interface/src/main/java/xsbti/compile/Setup.java

@@ -1,6 +1,8 @@
 package xsbti.compile;
 
 import java.io.File;
+import java.util.Map;
+
 import xsbti.Maybe;
 import xsbti.Reporter;
 
@@ -30,4 +32,16 @@ public interface Setup<Analysis>
 
 	/** The reporter that should be used to report scala compilation to. */
 	Reporter reporter();
+
+	/**
+	 * Returns incremental compiler options.
+	 *
+	 * @see sbt.inc.IncOptions for details
+	 *
+	 * You can get default options by calling <code>sbt.inc.IncOptions.toStringMap(sbt.inc.IncOptions.Default)</code>.
+	 *
+	 * In the future, we'll extend API in <code>xsbti</code> to provide factory methods that would allow to obtain
+	 * defaults values so one can depend on <code>xsbti</code> package only.
+	 **/
+	Map<String, String> incrementalCompilerOptions();
 }