Use "createTime" instead of "createDate"

[zorun]

I was getting "Invalid date" as creation date in the interface, because
the Javascript didn't seem to like the format of the dates returned by
BBB.

Switch to using the "createTime" attribute which is a standard Unix
timestamp. It avoids the need to parse dates in Javascript.

Tested with BBB 2.2

[greenstatic]

Thanks for the PR, like I mentioned in #4, I'll confirm this works as expected on tomorrow / sunday at the latest and make a new release.

[zorun]

Ok, thanks!

Note that I changed an API field instead of adding a new field, breaking backwards compatibility of the API. But at this stage it doesn't look like a big issue, correct me if I'm wrong.