Document (or automate) ./check.sh needing --pyargs bayeslite when using "$@" mode.

[fsaad]

Otherwise the user will be running ./check.sh against the source file and not the built version in build/. Using the source file instead of the build directory will result in package_data (such as csv files) that is not in setup.py to be used for testing, when in fact it they are not available in the build.