Add base_datetime property

[bemoody]

wfdb.rdrecord currently sets an undocumented attribute base_datetime, but only if the record has a base date and a base time.

>>> r = wfdb.rdrecord('sample-data/n16')
>>> r.base_time
datetime.time(22, 34, 47)
>>> r.base_date
datetime.date(2006, 1, 1)
>>> r.base_datetime
datetime.datetime(2006, 1, 1, 22, 34, 47)

>>> r2 = wfdb.rdrecord('sample-data/3000003_0003')
>>> r2.base_time
datetime.time(19, 46, 25, 757000)
>>> r2.base_date
>>> r2.base_datetime
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Record' object has no attribute 'base_datetime'

It is frequently convenient to have this attribute available, so:

1. Always set base_datetime; set it to None if either base_date or base_time is None.

2. Define it as an aliased property so that setting base_date and base_time implicitly sets base_datetime and vice versa.

3. For consistency, also accept base_datetime as an argument to wfdb.Record, wfdb.MultiRecord, and wfdb.wrsamp.

Also, fix the documentation that incorrectly claimed base_date and base_time were strings. They weren't.

[bemoody]

Oh, incidentally it looks like this is required in order for pull #380 to work correctly - I didn't even notice that. :)

[tompollard]

Looks good to me!