pd.read_sql_query() does not convert NULLs to NaN
#### A small, complete example of the issue

``` python
from sqlalchemy import create_engine
import pandas as pd

engine = create_engine('sqlite://')
conn = engine.connect()
conn.execute("create table test (a float)")
for _ in range(5):
    conn.execute("insert into test values (NULL)")

df = pd.read_sql_query("select * from test", engine, coerce_float=True)
print(df.a)
```
#### Expected Output

In pandas 0.18.1 this will result in a column of type `object` with `None` values, whereas I needed float("nan"). The coerce_float=True option made no difference. This is most needed, when reading in a float column _chunk-wise_, since there may be sequences of NULLs.

(also http://stackoverflow.com/questions/30652457/adjust-pandas-read-sql-query-null-value-treatment/)
