ENH: accept dict of column:dtype as dtype argument in DataFrame.astype
Continued in https://github.com/pydata/pandas/pull/13375

---

closes #7271

By passing a dict of {column name/column index: dtype}, multiple columns can be cast to different data types in a single command. Now users can do:

`df = df.astype({'my_bool', 'bool', 'my_int': 'int'})`

or:

`df = df.astype({0, 'bool', 1: 'int'})`

instead of:

```
df['my_bool'] = df.my_bool.astype('bool')
df['my_int'] = df.my_int.astype('int')
```
