add example of loading lib from ES2015 code

Author: tdumitrescu

build.sh

@@ -20,4 +20,5 @@ java -jar $COMPILER --js mixpanel-jslib-snippet.js --js_output_file mixpanel-jsl
 java -jar $COMPILER --js mixpanel-jslib-snippet.js --js_output_file mixpanel-jslib-snippet.min.test.js --compilation_level ADVANCED_OPTIMIZATIONS --define='MIXPANEL_LIB_URL="../mixpanel.min.js"'
 
 pushd examples/commonjs-browserify; npm run build; popd
+pushd examples/es2015-babelify; npm run build; popd
 pushd examples/umd-webpack; npm run build; popd