feat(select): add auto-inference of option value

This makes md-option follow HTML5 behavior of setting the value to
the text value of the option if no value is specified.

closes #1713

src/components/select/select.js

@@ -566,7 +566,8 @@ function OptionDirective($mdInkRipple, $mdUtil) {
     } else if (angular.isDefined(attr.value)) {
       setOptionValue(attr.value);
     } else {
-      throw new Error("Expected either ngValue or value attr");
+      debugger;
+      scope.$watch(function() { return element.text(); }, setOptionValue)
     }
 
     attr.$observe('selected', function(selected) {

src/components/select/select.spec.js

@@ -146,12 +146,20 @@ describe('<md-select-menu>', function() {
     }));
   });
 
+  it('auto-infers a value when none specified', inject(function($rootScope) {
+      $rootScope.name = "Hannah";
+      var el = setup('ng-model="name"', '<md-option>Tom</md-option>' +
+            '<md-option>Hannah</md-option>');
+      expect(selectedOptions(el).length).toBe(1);
+  }));
+
   it('errors for duplicate md-options, non-dynamic value', inject(function($rootScope) {
     expect(function() {
       setup('ng-model="$root.model"', '<md-option value="a">Hello</md-option>' +
             '<md-option value="a">Goodbye</md-option>');
     }).toThrow();
   }));
+
   it('errors for duplicate md-options, ng-value', inject(function($rootScope) {
     setup('ng-model="$root.model"', '<md-option ng-value="foo">Hello</md-option>' +
           '<md-option ng-value="bar">Goodbye</md-option>');