Allow installation timezone configuration (#44)

This allows setting the timezone as a configuration variable
`adhocracy.timezone`.

This variable is used in the helper functions `date_tag` and
`datetime_tag`. The latter was previously called `relative_time`.

This also adds a `<time>` html tag around pure date informations, i.e.
`h.format_date` has been replaced with `h.date_tag` all around.

This is the first part of issue #44. The dynamic javascript part remains
open.