Fix on build.xml, added clean target

QIR/README

@@ -2,4 +2,5 @@ This project is an implementation of the Query Intermediate Representation.
 
 == Build ==
 
-To create the archive qir.jar, run ant on the root directory.
+To create the archive qir.jar, run "ant" on the root directory.
+To remove generated files, run "ant clean".

QIR/build.xml

@@ -2,9 +2,15 @@
 <!-- Configuration of the Ant build system to generate a Jar file -->
 <project name="QIR" default="CreateJar">
   <target name="CreateJar" description="Create Jar file">
+    <mkdir dir="bin"/>
     <jar jarfile="qir.jar">
       <fileset dir="bin"/>
       <fileset dir="src" includes="**/*.java"/>
     </jar>
   </target>
+
+  <target name="clean" description="Removes generated files">
+    <delete dir="bin"/>
+    <delete file="qir.jar"/>
+  </target>
 </project>