Opens the speicified index.
        http://www.elasticsearch.org/guide/reference/api/admin-indices-open-close.html

        > ElasticSearch().index_open('my_index')

def index_open(self, index):
        '''
        Opens the speicified index.
        http://www.elasticsearch.org/guide/reference/api/admin-indices-open-close.html

        > ElasticSearch().index_open('my_index')
        '''
        request = self.session
        url = 'http://%s:%s/%s/_open' % (self.host, self.port, index)
        response = request.post(url,None)
        return response