Fix Studio's XBlock dependency loading issues

STUD-1465

Changed the logic so that each dependency must be fully loaded before the next one is started. This gives XBlock authors the same semantics in Studio as they get when viewing an XBlock directly on a page.

See this Stack Overflow answer for an overall explanation:
http://stackoverflow.com/questions/8026191/can-i-use-jquery-getscript-without-a-callback/8026693#8026693