add gitignore

xxl-job-core/.gitignore

 /.classpath
 /.project
 /.settings/
-/xxl-job-client.iml
+/xxl-job-core.iml
 /target/

xxl-job-executor-example/.gitignore

 /.classpath
 /.project
 /.settings/
-/xxl-job-client-demo.iml
+/xxl-job-executor-example.iml
 /target/