Add date and time type variable

[jachymb]

It would be good to have date and time types (using datetime.date and datetime.datetime internally) that would allow for comparisons using the less_than etc operators.

[trevoriancox]

My fork has After, Before, and Equal To for new FIELD_DATETIME and FIELD_TIME types. Feel free to use.

The fork is not in a state to do a pull request, because includes other changes:

• Treat no conditions as unconditional.
• Improved ajax support.

https://github.com/trevoriancox/business-rules