Fixes (re-adds) the processor interface as per consensus in Issue #22

Author: viktorklang

Diff for: api/src/main/java/org/reactivestreams/Processor.java

@@ -0,0 +1,8 @@
+package org.reactivestreams;
+
+/**
+ * A Processor represents a processing stage—which is both a {@link Subscriber}
+ * and a {@link Publisher} and obeys the contracts of both.
+ */
+public interface Processor<T, R> extends Subscriber<T>, Publisher<R> {
+}