fix bug: select statement with where syntax without priority

当执行查询时, 查询条件中存在优先级的情况下, 无法处理优先级的情况.

    import CURD
    CURD.Database.config(user='root',passwd='asd',db='curdtest')
    import models
    User=models.User
    User(name='Peer.Xu',email='just.for.fun@gmail.com').save()
    assert len(list(User.where((User.id<0)&((User.name=='Peer.Xu')|(User.email=='just.for.fun@gmail.com'))).getall())) == 0