Series.drop() with boolean index: inconsistent behaviour
The behaviour below occurs in versions `'0.15.0rc1-21-g32c5016'` and `'0.14.1'`.

When the `label` passed to the `drop` method of a `Series` is not in the index:
(a) if the index is not all booleans then an error is raised
(b) if the index is all booleans and has length > 1 then no error is raised; the original series is returned
(c) if the index is all booleans and has length == 1 then an error is raised

I propose that:
1. the difference between the behaviour in (a) and (b) should be documented
2. the behaviour in (c) should be changed to match that in (b)

Examples of current behaviour:
(a)

``` python
>>> pd.Series([1, 1], index=['a', True]).drop(False)
ValueError: labels [False] not contained in axis
```

(b)

``` python
>>> pd.Series([1, 1], index=[True, True]).drop(False)
True    1
True    1
dtype: int64
```

(c)

``` python
>>> pd.Series([1], index=[True]).drop(False)
ValueError: labels [False] not contained in axis
```
