Create several datetimepickers with class selector

Fixes Issue #201 by moving _xdsoft_datetime declaration from
$.fn.datetimepicker() down into its createDateTimePicker()

With this fix, the next/prev month arrows work in all datetimepickers,
and not just the last.
Merge pull request #219 from bobmazanec/issue-201-create-several-by-class

Create several datetimepickers with class selector