system-test: support blocking rewrites (fixes #199)
Merge pull request #200 from pagespeed/jefftk-blocking-rewrite-key

system-test: support blocking rewrites
Merge branch 'master' into jefftk-system-test-content-handler-priority
system-test: clear cache; simpler init

* always run with a clean cache
 * in the future we should structure the tests so they run twice, once with a clean cache and then again with a populated one, but for now a clean cache should catch more bugs
* stop requiring a file cache path for tests; just use
* fix exit codes to match documentation at the top of the file: 1 is a test failure, 2 is bad args

Squash-merge of my #198.
Merge branch 'master' into jefftk-system-test-content-handler-priority
system-test: FILE_CACHE_PATH -> FILE_CACHE