Add getTimezone method

It's useful to retrieve the standard timezone ID starting from 'user',
for instance...
PRS-2 coding style
Fix date/time widget

- Take in account the correct timezone
- Don't use javascript timestamps (working in JS with timestamp and timezone is quite hard)
- Avoid posting <value> and <value_pub>: jist <value> please ;)
- Allow specifying empty values for initial date/time widget
Bugfix
Merge pull request #670 from mlocati/fix-date-input

Fix date/time widget