Parse and convert firstaired dates to datetime.datetime() objects

[poxip]

The firstaired dates could be parsed and converted to Python datetime.datetime() objects, which's sometimes helpful a bit.

[dbr]

In the old XML API, the air-dates were an arbitrary string so was impossible to reliably parse, hopefully the new JSON'y API returns the dates in a consistent and this should be easy

[dbr]

The JSON API appears to give the datestamp in a consistent format, 2001-10-16, so this should be doable

This would break the API compatibility quite a bit so would likely have to wait till a future revision