Add gems to run rspec tests
Install rspec's rake tasks
bundle exec rspec --init
Create rails app for testing
Add datetime_picker method to render datetime picker
Add datetimepicker_class option to specify wrapping div's class

The Default value is "bootstrap_form-datetimepicker".
Update specs to enable template to detect schedule instance

FormBuilder's methods use instance variables' names to know their
values, but rspec's let prevents it.
Add spec to rendering fiels with values
Add format_name option to specify Time::DATE_FORMAT's key to put as value attribute
Use format specified with format_name option as data-date-format attribute
Change default data-date-format to rails' default format