Add test data to emperor -- fixes issue 44

`all_tests.py` has been updated to make use of the test data repository so the script usage tests can be executed.

Note that given that the test data directory is not deployed when `setup.py` is executed to install Emperor, `all_tests.py` offers options to specify the folder where the test_data directory is, as well as the scripts directory. This is different to QIIME because when QIIME's `setup.py` is executed the test_data directory is also copied into the user system.