ENH/SQL: provide single value to 'dtype' argument in to_sql as dtype for all columns
Idea from http://stackoverflow.com/questions/34383000/pandas-to-sql-all-columns-as-nvarchar

You can specify the sql type using the `dtype` argument in `to_sql`. Now, this argument has to be a dict (mapping of column name -> type). 
Letting `to_sql` recognize a single value as the sql type for all columns is not that difficult to add.

So instead of this:

```
df.to_sql(...., dtype={col_name: sqlalchemy.types.NVARCHAR for col_name in df})
```

you can do this:

```
df.to_sql(...., dtype=sqlalchemy.types.NVARCHAR)
```
