API: resample with PeriodIndex default span (start/end convention)
Previously, for resampling with PeriodIndex, you had two conventions: `'start'` (start -> start) and `'end'` (end -> end). This would give something like this (note: the following is **not current real code output**, but from Wes' book):

```
In [25]: s = pd.Series(np.arange(2), index=pd.period_range('2000-1', periods=2, freq='A'))

In [26]: s
Out[26]:
2000    0
2001    1
Freq: A-DEC, dtype: int32

In [27]: s.resample('Q-DEC', fill_method='ffill', convention='start')
Out[27]:
2000Q1    0
2000Q2    0
2000Q3    0
2000Q4    0
2001Q1    1
Freq: Q-DEC, dtype: int32

In [28]: s.resample('Q-DEC', fill_method='ffill', convention='end')
Out[27]:
2000Q4    0
2001Q1    1
2001Q2    1
2001Q3    1
2001Q4    1
Freq: Q-DEC, dtype: int32
```

Following Wes' book, the default argument was 'end'. However, the current behaviour is like this (this is real output):

```
In [27]: s.resample('Q-DEC', fill_method='ffill')
Out[27]:
2000Q1    0
2000Q2    0
2000Q3    0
2000Q4    0
2001Q1    1
2001Q2    1
2001Q3    1
2001Q4    1
Freq: Q-DEC, dtype: int32
```

So in fact this is a third option `'span'` (start -> end). This option is mentioned in #1635, but from the issue it seems it was never implemented ([the commit](https://github.com/changhiskhan/pandas/commit/cb81f9ad5607419a5d11ceafd2225c2fdf0e6632) was never merged. There was a test added in comments at that time, but this is still in comments: https://github.com/pydata/pandas/blob/master/pandas/tseries/tests/test_resample.py#L1134). 
In practice, however, this is the case (the default behaviour is this mentioned 'span' behaviour). But also the option `'start'` has changed:

```
In [28]: s.resample('Q-DEC', fill_method='ffill', convention='start')
Out[28]:
2000Q1    0
2000Q2    0
2000Q3    0
2000Q4    0
2001Q1    1
2001Q2    1
2001Q3    1
2001Q4    1
Freq: Q-DEC, dtype: int32
```

This gives the same as the default (only for `'end'` it is the same as before).

Some issues/questions:
- what is the default value for `convention`? It is nowhere in the  [docs](http://pandas.pydata.org/pandas-docs/stable/timeseries.html#up-and-downsampling), and also not in the [docstring](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.resample.html) (apart from the signature, which says 'start').
- I don't find the issue/PR/release note where it says that the default for period resample (upsampling) has changed
- the default now is a 'spanning' behaviour, but this is the same as 'start'. Shouldn't be this something else? So that the 'start' option has another behaviour (start -> start) than the default spanning behaviour ('start' -> 'end')?
