Add support to update jobs, if they exist.

Added an optional argument to create_copy(), to indicate whether to
update the job if it already exists.
Use the argument name _force instead of update.

In create_copy(), use the argument name _force, instead of update, to
avoid collision with other template variable names. Also added a unit
test case to cover this scenario.
Merge pull request #23 from bravegnu/master

Add support to update jobs, if they exist.