Fixed pot. NPE logging LTI domain launch failures

The audit logger expects the Delivery to be bound to an Assessment,
which is not the cause if an LTI resource has not been set up correctly.
I've added a check to deal with this.
Fixed #52: MathJax CDN HTTPS URL has changed

This commit fixes the URL for the MathJax CDN (over HTTPS) to use the
new URL that appears to have been introduced recently.

The old URL is no longer working, so you are advised to apply this fix
ASAP if you need MathML working.
Fixed #55: Wrong attr in BodyElement.[gs]etLabel()

This method was acting on the 'lang' variable instead of 'label',

Thanks to @ccreel for the bug report.
Slightly improved WS/REST documentation (#56)

I've tidied up the documentation for SimpleRestRunner and added a
pointer to this in default.qtiworks-deployment.properties. This is in
response to issue #56.