Period.end_time should be exclusive or Period should provide duration property
A user may expect that the end_time of a Period minus the start_time of a Period returns its duration. Unfortunately, the end_time of a Period is inclusive, consequently, subtracting start from end yields an incorrect period duration:

```
jan = Period('2014-01', freq='M')
(jan.end_time - jan.start_time).days # returns 30, not 31
```

This can be manually rectified by the user:

```
jan = Period('2014-01', freq='M')
(Timestamp(jan.end_time.value + 1) - jan.start_time).days # correctly returns 31
```

However, this is not intuitive.

The end_time should either be exclusive or Period should provide a duration property.

Although a strong argument can be made that the definitions of duration, start_time, and end_time should all be consistent, changing the end_time definition to exclusive is a significant modification of long-standing behavior.

Consequently, I propose the addition of a duration property that is logically inconsistent with subtracting start_time from end_time.
