Support handling undefined and null

[ahumphreys87]

The following test case shows the use of passing undefined and null into the compiler.

  it('should handle undefined and null', function() {
    shouldCompileTo('{{awesome undefined null}}',
        {
          awesome: function(_undefined, _null, options) {
            return (_undefined === undefined) + ' ' + (_null === null) + ' ' + (typeof options);
          }
        },
        'true true object');
    shouldCompileTo('{{undefined}}',
        {
          'undefined': function() {
            return 'undefined!';
          }
        },
        'undefined!');
    shouldCompileTo('{{null}}',
        {
          'null': function() {
            return 'null!';
          }
        },
        'null!');
  });