fix ignore & replace syntax for mysql
Currently ran into this. Would be awesome to see 1`INSERT IGNORE INTO` added for MySQL @coleifer what would need to happen to get this merged in. Would be willing to create new PR if needed. This should be working in 3.x. See:

* http://docs.peewee-orm.com/en/latest/peewee/api.html#Insert
* http://docs.peewee-orm.com/en/latest/peewee/api.html#Model.replace
2 bugs in peewee.py
1.   `upsert_sql = 'INSERT OR REPLACE INTO'`   will have syntax error for Mysql. So I fix it like `upsert_sql = 'REPLACE INTO'`.
2.  add ignore for mysql   `INSERT OR IGNORE INTO` will have syntax error for Mysql.

Example:
Page.insert(data).ignore().execute()
Page.insert_many(data).ignore().execute()

Generate sql：
'INSERT IGNORE INTO `page` (`manga_id`, `page_num`, `img_url`) VALUES (%s, %s, %s)', [11997, 6, 'http://img.177pic66.com/uploads/2016/05a/00178.jpg']

And you will get mysql warning when you use `ignore` but it can work：

```
/Users/wyx/project/scrap/.env/lib/python3.5/site-packages/pymysql/cursors.py:166: Warning: (1062, "Duplicate entry '6' for key 'page_page_num'")
  result = self._query(query)
```
