use date and time pickers

- use two separate fields for date and time
- add date picker from https://github.com/eternicode/bootstrap-datepicker and
  time picker from https://github.com/weareoutman/clockpicker
- caffeine.forms.SubmitCaffeineForm:
  - add date and time fields
  - move validation code to clean method and combine date and time fields to
    combined date field of Caffeine model instance
- caffeine/js/validation.js:
  - use strict mode
  - add separate coffeedate and coffeetime functions
  - split datetimepat to datepat and timepat
  - change sanitize_datetime to validate a date and a time field together
- templates/base.html:
  - reference datepicker and timepicker CSS and JavaScript
  - add markup for clockpicker and datepicker
  - add jQuery code for clockpicker and datepicker fields