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 - I went back and forth on that for awhile. The nice thing it'd give us is test discovery and the framework for running them all. The thing we likely wouldn't need are the pass/fail nature of the tests, since correctness isn't what should be tested by these tests - only performance and timing.

If I did this, then I should probably move the test to be mixed in with the regular unittests? Maybe in a test_perf_*.py file?