Fixing format time issue

I did not realise that the Redmine format_time method also formats for
time zone. This breaks the working periods as the time zone offset gets
added twice.
Also fixed similar issue in the pause model (although this wasn't
causing a bug because calling in_time_zone twice doesn't apply the
offset twice)
Merge pull request #70 from dannybutler/format_time_fix

Fixing format time issue