fix(alert): use interpolation for type attribute

Closes #1460

BREAKING CHANGE: Use interpolation for type attribute.

  Before:

  ```html
  <alert type="'info'" ...></alert >
  ```
  or
  ```html
  <alert type="alert.type" ...></alert >
  ```

  After:

  ```html
  <alert type="info" ...></alert >
  ```
  or
  ```html
  <alert type="{{alert.type}}" ...></alert >
  ```

src/alert/alert.js

@@ -12,7 +12,7 @@ angular.module('ui.bootstrap.alert', [])
     transclude:true,
     replace:true,
     scope: {
-      type: '=',
+      type: '@',
       close: '&'
     }
   };

src/alert/docs/demo.html

@@ -1,4 +1,4 @@
 <div ng-controller="AlertDemoCtrl">
-  <alert ng-repeat="alert in alerts" type="alert.type" close="closeAlert($index)">{{alert.msg}}</alert>
+  <alert ng-repeat="alert in alerts" type="{{alert.type}}" close="closeAlert($index)">{{alert.msg}}</alert>
   <button class='btn btn-default' ng-click="addAlert()">Add Alert</button>
 </div>

src/alert/test/alert.spec.js

@@ -13,7 +13,7 @@ describe('alert', function () {
 
     element = angular.element(
         '<div>' +
-          '<alert ng-repeat="alert in alerts" type="alert.type"' +
+          '<alert ng-repeat="alert in alerts" type="{{alert.type}}"' +
             'close="removeAlert($index)">{{alert.msg}}' +
           '</alert>' +
         '</div>');
@@ -88,7 +88,7 @@ describe('alert', function () {
   });
 
   it('should be possible to add additional classes for alert', function () {
-    var element = $compile('<alert class="alert-block" type="\'info\'">Default alert!</alert>')(scope);
+    var element = $compile('<alert class="alert-block" type="info">Default alert!</alert>')(scope);
     scope.$digest();
     expect(element).toHaveClass('alert-block');
     expect(element).toHaveClass('alert-info');