ENH/API: Allow get_dummies to specify return value
Currently `get_dummies` always returns dummy values with `0` or `1`. When we want others,  have to use `replace`, `apply...` etc (and it is not easy in some cases because column name can differs depending on target value).

```
s = pd.Series(['A', 'A', 'B', 'C'])
pd.get_dummies(s)
#    A  B  C
#0  1  0  0
#1  1  0  0
#2  0  1  0
#3  0  0  1
```

Allow to specify values to be used with `values` and `na_value` kwds corresponding `True` or `False`, accepting `scalar`, `dict`, or `Series` like `fillna` does.

```
pd.get_dummies(s, value=True, na_value=False)
#        A      B      C
#0   True  False  False
#1   True  False  False
#2  False   True  False
#3  False  False   True
```
