Set the local time zone as the default

This change allows the `TimePointParser` class to cast date/time
representations with unknown time zones into the local
time zone by default (although there are two switches,
`assume_utc` and `assume_unknown_time_zone` that switch off
this behaviour). Previously, they became UTC by default, which
goes against the spirit of the standard.

This change should be merged after #42.

@arjclark, please review.