fix: Increase testing coverage of core extensions
fix Date(Time) and Time core-extension types

- always use UTC for Date and Time (DateTime still allows configuring)
- use a strict type-hint for date(Time) types
add BirthdayTypeTest
fix ChoiceType working with objects
add additional tests for core-types
always use UTC for the TimeType
fix missing error-message in TransformFormatter
bug #30 fix broken core-extension types (sstok)

This PR was merged into the master branch.

Discussion
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|Q            |A  |
|---          |---|
|Bug Fix?     |yes|
|New Feature? |no |
|BC Breaks?   |yes |
|Deprecations?|no |
|Fixed Tickets| #28  |
|License      |MIT|

The only BC is that Time now always uses UTC. but using a timezone was broken anyway in this case.

Commits
-------

e950d0751b8144700d9f6d47f4fff7590c4b6019 fix: Increase testing coverage of core extensions
12fb86fff0132061b96e0c96b0036ad674ce87f2 fix Date(Time) and Time core-extension types
71848f60bf8b39c981655a147ece79ac4852df12 add BirthdayTypeTest
ed86c14855ac7f817b9f4eff32e08fdaf32f940a fix ChoiceType working with objects
a33f82f07e055682314a86c7ca6c16adcd2ae062 add additional tests for core-types
c59347435845843b5e4bd01632f5e981afaddbdb always use UTC for the TimeType
4c1c084b9220b189afcbd33bf0c509b4c0c0c7e7 fix missing error-message in TransformFormatter