feat(timepicker): added support for seconds

timpicker can now show seconds as well which is by default hidden. passing `timepickerConfig.showSeconds=true` or `<timepicker show-seconds="true" ...>` will enable this feature. Additional tests and documentation is still missing, and thats why I call to volunteers to help with adding tests and documentation particularly in https://github.com/angular-ui/bootstrap/issues/2326 https://github.com/angular-ui/bootstrap/issues/1763 https://github.com/angular-ui/bootstrap/issues/1231