fix timestampable use of datetimeclass

[smhg]

The timestampable behavior doesn't use the generator.dateTime.dateTimeClass setting.
I took a few liberties adressing this:

• Reusing the getDateTimeClass method of Propel\Generator\Builder\Om\ObjectBuilder. For this it had to become public.
• PropelDateTime::createHighPrecision expects the output of microtime to be formatted with a dot. This places the formatting in a separate (static) method.
  This allows keeping the updated_at and created_at microtimes in preInsert exactly the same value (instead of generating them independently).

I've avoided simplifying code to not break things.

What do you think?

[smhg]

@dereuromark based on other PR's it seems the CI errors where not introduced here. I think it is better to address at them in a separate PR, no?

What do you think about this change?

[smhg]

I get little notifications of GitHub myself, so tagging @PhilinTv and @vol4onok.