Added session lifetime settings

- default session lifetime settings
- configurable in admin area
specs for session lifetime before_filter

- only added the tests to the users_controller
- TODO move the test to a proper place
[opf1054] Implements configurable session_lifetime

- added locales also for the settings
more reliable session lifetime

- allow only integer value for session ttl
- disable session lifetime for ttl < 5 min
- added hint to settings
changed wording; spec new case
Merge pull request #225 from opf/feature/rails3_add_configurable_session_lifetime

Feature/rails3 add configurable session lifetime