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.
Yeah, it would mainly aid discovery and execution (but I agree, there's not really any correctness, so it's still a bit odd). I'd probably not put it in w/ the other tests (instead, I might put it in a `perf_test` folder adjacent to `test`).

@jzoldak, @cgoldberg: Any comments on this?