sqlalchemy example doesn't save rows

I was following https://github.com/aio-libs/aiomysql/blob/master/docs/sa.rst and I noticed that running the example multiple times did not work as expected.

The example:
```
import asyncio
import sqlalchemy as sa

from aiomysql.sa import create_engine

metadata = sa.MetaData()

tbl = sa.Table('tbl', metadata,
sa.Column('id', sa.Integer, primary_key=True),
sa.Column('val', sa.String(255)))

@asyncio.coroutine
def go():
engine = yield from create_engine(user='root',
db='dabbot',
host='db',
password='dabbot')

with (yield from engine) as conn:
yield from conn.execute(tbl.insert().values(val='abc'))

res = yield from conn.execute(tbl.select())
for row in res:
print(row.id, row.val)

asyncio.get_event_loop().run_until_complete(go())

```

```
bash-4.4$ python /app/simple.py
1 abc
bash-4.4$ python /app/simple.py
2 abc
bash-4.4$ python /app/simple.py
3 abc
bash-4.4$ python /app/simple.py
4 abc
bash-4.4$ python /app/simple.py
5 abc
```

Then I opened up adminer and added `6 def`

```
bash-4.4$ python /app/simple.py
6 def
7 abc
bash-4.4$ python /app/simple.py
6 def
8 abc
```

I think the example is missing `conn.execute('commit')`.

The example at https://github.com/aio-libs/aiomysql/blob/master/examples/example_simple_sa.py has the same issue. I removed the "drop" statement and it doesn't persist rows like I expect.
```
bash-4.4$ python /app/simple.py
1 abc
2 xyz
bash-4.4$ python /app/simple.py
3 abc
4 xyz
bash-4.4$ python /app/simple.py
5 abc
6 xyz
```