WIP: Performance testing of asset metadata in course import/export

First iteration of code which tests performance of asset metadata during course import/export. Uses external code_block_timer module. Also, add script which validates XML file against an XSD file.
I just saw pointers to https://github.com/joke2k/faker (first I'd heard of it). Not sure if it would give you better data, or give you a better way of generating data.