Suggestion: a dtype argument for read_sql
The `pandas.io.sql` module has a convenient `read_frame` function which has been of great use to me. The function has an argument `coerce_float`. I propose to make that more general akin to the `pandas.DataFrame` initialization method which has an argument `dtype`. This would allow passing a `dict` with column names as keys and the desired data type as values. Columns not in the `dict` would be inferred normally.

If this is not planned but desired I can look into patching that myself.
