Epoch (%s format) support proposal

Here are my proposal, tested with Postgis, SpatialLite, Shapefiles data : support of the %s format, (minus 's', like some api). This allows using numeric fields as timestamp values.

Changes in "timelayermanager", "timemanager", "timemanagercontrol" concern the new "$animation_epoch" variable, which can be renamed of course.

timevectorlayer script :
- as datetime object does not implements "totimestamp" function, here we implement it.
- a "datetimeToStr" function has been added
- close to line 120, minValue and maxValue management have been separated

later, I'll send you the second change, about the toTimeAttribute managed as fixed numeric value...