Make TimeWithZone work with tzinfo 0.2.x: use TZInfo::Timezone#zone_identifier alias for #abbreviation, silence warnings on tests. Raise LoadError when TZInfo version is < 0.2 by sniffing for TZInfo::TimeOrDateTime constant. Move all tzinfo-dependent TimeZone tests into uses_tzinfo block

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@9071 5ecf4fe2-1ee6-0310-87b1-e25e094e27de