Changed make_aware() to use the current timezone by default

When calling `make_aware()`, either `get_current_timezone()` or `get_default_timezone()` seem to be a logical default. In my opinion, `get_current_timezone()` is seems more correct most of the time. That is, when handling a naive datetimes, converting it to the current time zone is the most typical operation. This change makes using `make_aware()` easier to use as it will now generally do the right thing by default. If the user wants to use a different timezone, the option is still available.
The changes need to be in a `.. versionchanged:: 1.8` block.