Fix/datetime parser

[imprologic]

Timezone parsing is not yet implemented in Dart's DateFormat class, see https://pub.dev/documentation/intl/latest/intl/DateFormat-class.html (scroll to "The following characters are reserved and currently are unimplemented").

This PR tries to fix this issue by adding a timezone parsing function, capable of:

• parsing the most common timezone abbreviations, like GMT, EST, PST etc.
• parsing timezone offsets like +0100 or -05:00.