[#7] A basic TimeRangeSliderField for wtforms.

Actually, as I know, we can not submit data directly via jquery ui
slider, so we create two hidden fields, for start_time and end_time,
respectively, then update the values for start_time and end_time fields
via JavaScript when user slider the handler.

Todo:
1. some frontend work to make it looks better
2. backend logic to make it work as expected.