Add option for custom date time format

Found another bug where it would still validate when leaving the end-date empty.

ps. an interdiff doesn't seem to generate : interdiff: Whitespace damage detected in input

Found another small issue where it would strip the time from the entered values. And the end-value wasn't handled correctly.

Here's a new patch.

The new option to configure a custom format per field widget works as expected, thanks!

@idebr, Thanks for the review. Why did you hide the patch from #23 and #16. Are you saying the one you tested is the last one not hidden (#14) ?

Please clarify and hide all files other than the one you tested, please..

I was unaware I changed the displayed patches. I tested the latest patch in #23.

Attached patch implements the following changes:

1. Applied default value formatting to single_date_time_timestamp_widget and single_date_time_widget
2. Applied massageFormValues() to all widgets that extend SingleDateTimeBase

I tested patch #28, looks good the default_value formatting is now correct.

Rerolled the latest patch for 2.0.0-beta2.