update(demo): Basic Input demo fix to floating label

BREAKING CHANGE: md-input-containers no longer allow both label and placeholder to be used simultaneously. Now the placeholder value (if present) is transcluded as a label or ignored.
When the placeholder value is ignored, a warning will be logged.

src/components/input/demoBasicUsage/index.html

@@ -28,8 +28,8 @@
 
       <div layout layout-sm="column">
         <md-input-container flex>
-          <label>First Name</label>
-          <input ng-model="user.firstName" placeholder="Placeholder text">
+          <label>First name</label>
+          <input ng-model="user.firstName">
         </md-input-container>
         <md-input-container flex>
           <label>Last Name</label>

src/components/input/input.js

@@ -326,7 +326,7 @@ function mdMaxlengthDirective($animate) {
   }
 }
 
-function placeholderDirective() {
+function placeholderDirective($log) {
   return {
     restrict: 'A',
     require: '^^?mdInputContainer',
@@ -341,10 +341,14 @@ function placeholderDirective() {
     var placeholderText = attr.placeholder;
     element.removeAttr('placeholder');
 
-    var placeholder = '<label ng-click="delegateClick()">' + placeholderText + '</label>';
+    if ( inputContainer.element.find('label').length == 0 ) {
+      var placeholder = '<label ng-click="delegateClick()">' + placeholderText + '</label>';
 
-    inputContainer.element.addClass('md-icon-float');
-    inputContainer.element.prepend(placeholder);
+      inputContainer.element.addClass('md-icon-float');
+      inputContainer.element.prepend(placeholder);
+    } else {
+      $log.warn("The placeholder='" + placeholderText + "' will be ignored since this md-input-container has a child label element.");
+    }
 
   }
 }