Adds an async batch fetch function for getting Canvas Pages.

[mbusch3]

Added a new apiGetBatch function to CanvasApi.php. It allows you to enter an array of URLs that need to be fetched and uses the curl_multi_* functions to create and watch all the handles and return once they've all gotten responses.

I've applied it to the updateCourseContent function in CanvasLms.php for pulling all the Page content from Canvas. On my own course that has 43 pages that need to be fetched, running a Full Course Rescan gave me the following times (in seconds):

Method	Test 1	Test 2	Test 3	Test 4	Test 5	Test 6	Average
Old/Sequential	9.504	10.286	9.633	9.914	9.529	10.185	9.842
New/Async	6.540	6.689	6.895	6.734	6.623	6.584	6.678

Obviously, the number and size of course Pages may drastically change your personal results.

I've kept all the testing code (including timers and readout) and both fetch methods so you can test it on different courses yourself. Just change the $asyncFetch = true; to false in the updateCourseContent function.