new edit widget for date/time capable of setting null dates

This adds a new QgsDateTimeEdit which inherits from QDateTimeEdit.
It handles and can set NULL dates.
The idea is the same than with QLineEdit: you have a "clear" icon on the right which will set it to NULL.

Here is how it looks "normally":
![image](https://cloud.githubusercontent.com/assets/127259/4091682/170e9ec8-2f8b-11e4-90ba-712b3c45f3ca.png)

when the date is NULL:
![image](https://cloud.githubusercontent.com/assets/127259/4091678/0467a77e-2f8b-11e4-8dfd-97d8802533ee.png)

There is an option in the config of the field to determine if it accepts NULL values or not.

If you design your own forms, this will be only accessible for those using the custom widgets (it will be included as soon as this PR goes through).

I hope you like it!