Adding support for saving the datetime internally as unix timestamp

this commit extends the datetime field with an option to save the choosen date as a unix timestamp in the databases metatable

it will try to convert the dates according to the *Format settings in field['js_option']. this functionality isn't perfect, because php's internal functions and jquery's are using different format strings. 
because strptime/mktime didn't work as expected on my system i ended up using DateTime::createFromFormat which is only available with php >5.3