fail when 'database' is used as connection setting

Connecting with _'database'_ as a connection setting (instead of _'db'_), will raise an error down the streaming line

``` python
settings = {
'host': 'localhost',
'port': 3306,
'user': 'root',
'passwd': '',
'database': 'stuff',
}
stream = BinLogStreamReader(connection_settings=settings, server_id=42)

for event in stream:
event.dump()
```

Will raise the following error in _TableMapEvent_

**pymysql.err.ProgrammingError: (1146, u"Table 'stuff.columns' doesn't exist")**

It seems that _'db'_ is overwritten internaly, but PyMysql will favor _'database'_ when present

Start at BinLogStreamReader and follow the connection settings into TableMapEvent, using the reported example as the reproduction. Check how the 'db' and 'database' settings are handled by PyMySQL and the replication code. Done means the example connects with 'database' and processes events without the TableMapEvent lookup error.

Written by the indexing model from the issue text.