Only include essential files in the generated XPI

.jpmignore

@@ -0,0 +1,22 @@
+/*.xpi
+/.git
+/.gitignore
+/.jpmignore
+/.jsbeautifyrc
+/.jshintrc
+/assets
+/CONTRIBUTING.md
+/design
+/doc
+/hooks
+/images
+/index.html
+/javascripts
+/LICENSE
+/node_modules
+/params.json
+/README.md
+/RELEASING.md
+/saucetesting
+/stylesheets
+/test