fix(sticky-headers): add feature detection

Closes #290

ionic/util/feature-detect.ts

@@ -0,0 +1,51 @@
+
+export class FeatureDetect {
+
+  run(window, document) {
+    this._results = {};
+    for (let name in featureDetects) {
+      this._results[name] = featureDetects[name](window, document, document.body);
+    }
+  }
+
+  has(featureName) {
+    return !!this._results[featureName];
+  }
+
+  static add(name, fn) {
+    featureDetects[name] = fn;
+  }
+
+}
+
+let featureDetects = {};
+
+
+FeatureDetect.add('positionsticky', function(window, document) {
+  // css position sticky
+  let ele = document.createElement('div');
+  ele.style.cssText = 'position:-webkit-sticky;position:sticky';
+  return ele.style.position.indexOf('sticky') > -1;
+});
+
+
+FeatureDetect.add('hairlines', function(window, document, body) {
+  /**
+  * Hairline Shim
+  * Add the "hairline" CSS class name to the body tag
+  * if the browser supports subpixels.
+  */
+  let canDo = false;
+  if (window.devicePixelRatio >= 2) {
+    var hairlineEle = document.createElement('div');
+    hairlineEle.style.border = '.5px solid transparent';
+    body.appendChild(hairlineEle);
+
+    if (hairlineEle.offsetHeight === 1) {
+      body.classList.add('hairlines');
+      canDo = true;
+    }
+    body.removeChild(hairlineEle);
+  }
+  return canDo;
+});