feat(theme,layout): support disabling of themes and layouts globally

* use `md-themes-disabled` attribute directive to disable themes:
stylesheet generation and DOM in injection
* use `md-layouts-disabled` attribute directive to disable layouts:
attribute-to-classname conversions are not performed
* update(layout, theme): use querySelector to find first only

Author: ThomasBurleson

src/core/services/layout/layout.js

@@ -101,8 +101,25 @@
 
     // Register other, special directive functions for the Layout features:
     module
-      .directive('mdLayoutCss'  , disableLayoutDirective )
-      .directive('ngCloak'      ,  buildCloakInterceptor('ng-cloak'))
+
+      .provider('$$mdLayout'     , function() {
+        // Publish internal service for Layouts
+        return {
+          $get : angular.noop,
+          validateAttributeValue : validateAttributeValue,
+          validateAttributeUsage : validateAttributeUsage,
+          /**
+           * Easy way to disable/enable the Layout API.
+           * When disabled, this stops all attribute-to-classname generations
+           */
+          disableLayouts  : function(isDisabled) {
+            config.enabled =  (isDisabled !== true);
+          }
+        };
+      })
+
+      .directive('mdLayoutCss'        , disableLayoutDirective )
+      .directive('ngCloak'            , buildCloakInterceptor('ng-cloak'))
 
       .directive('layoutWrap'   , attributeWithoutValue('layout-wrap'))
       .directive('layoutNowrap' , attributeWithoutValue('layout-nowrap'))
@@ -126,7 +143,10 @@
       .directive('hideLtMd'       , warnAttrNotSupported('hide-lt-md'))
       .directive('hideLtLg'       , warnAttrNotSupported('hide-lt-lg'))
       .directive('showLtMd'       , warnAttrNotSupported('show-lt-md'))
-      .directive('showLtLg'       , warnAttrNotSupported('show-lt-lg'));
+      .directive('showLtLg'       , warnAttrNotSupported('show-lt-lg'))
+
+      // Determine if
+      .config( detectDisabledLayouts );
 
     /**
      * Converts snake_case to camelCase.
@@ -143,6 +163,21 @@
 
   }
 
+
+  /**
+    * Detect if any of the HTML tags has a [md-layouts-disabled] attribute;
+    * If yes, then immediately disable all layout API features
+    *
+    * Note: this attribute should be specified on either the HTML or BODY tags
+    */
+   /**
+    * @ngInject
+    */
+   function detectDisabledLayouts() {
+     var isDisabled = !!document.querySelector('[md-layouts-disabled]');
+     config.enabled = !isDisabled;
+   }
+
   /**
    * Special directive that will disable ALL Layout conversions of layout
    * attribute(s) to classname(s).
@@ -164,13 +199,12 @@
    *
    */
   function disableLayoutDirective() {
+    // Return a 1x-only, first-match attribute directive
+    config.enabled = false;
+
     return {
       restrict : 'A',
-      priority : '900',
-      compile  : function(element, attr) {
-        config.enabled = false;
-        return angular.noop;
-      }
+      priority : '900'
     };
   }