Make datetime.datetime.today() return a FakeDateTime with no timezone info

...rather than a FakeDate. datetime.datetime.today() is effectively an alias
for datetime.datetime.now(tz=None), but freezegun was returning a FakeDate
object instead of a FakeDateTime.
Merge pull request #42 from robgolding63/shadow-today-api

datetime.datetime.today() should return a FakeDateTime