Figure out how to avoid manual updating of tzinfo database

[kaikuchn]

It is annoying that we have to watch for TZInfo Database releases and then wait for a TZInfo gem update to finally update this gem.
It also means that we potentially cause mistakes until the new database release is incorporated.

Elixir's excellent Timex library has a way better approach to handling this, for them the TZInfo-DB is just data and not part of the release.

I'd like to offer such an auto-fetching of new tzinfo db releases as opt-in. This way users would still retain full control over which version of the tzinfo db is in use.

Anyway, this would probably require us to no longer rely on the tzinfo gem, which is a lot of work..