Add gulpfile.js

+const srcPath   = './lib/relative-placement.js';
+const distPath  = './dist/';
+var gulp    = require('gulp');
+var babel   = require('gulp-babel');
+var replace = require('gulp-replace');
+var rename  = require('gulp-rename');
+var del     = require('del');
+var package = require('./package.json');
+gulp.task('default', ['build']);
+gulp.task('build',['clean'], ()=>{
+    return gulp.src(srcPath)
+        .pipe(replace(/RelativePlacement.version = '.+?';/, 'RelativePlacement.version = \''+package.version+'\';'))
+        .pipe(gulp.dest(distPath))
+        .pipe(babel({
+            presets: ['es2015']
+        }))
+        .pipe(rename(path => {
+            path.basename += '-es2015'
+        }))
+        .pipe(gulp.dest(distPath));
+});
+gulp.task('clean',() => {
+    return del(distPath);
+});
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