BUG: Exception raised when `to_sql` used to insert empty dataframe

Right now, if inserting an empty table (no rows) using `to_sql`, the following exception is raised:
```
...
  File "/Users/artemy/dev/pandas/pandas/io/sql.py", line 1220, in to_sql
    table.insert(chunksize)
  File "/Users/artemy/dev/pandas/pandas/io/sql.py", line 659, in insert
    chunks = int(nrows / chunksize) + 1
ZeroDivisionError: division by zero
```

This fixes this bug and adds tests for it.