merge qiime_test_data into qiime

This addresses #590 by merging the qiime_test_data repository into qiime. 

Note: After merging, users will no longer have to specify ``qiime_test_data_dir`` in their ``qiime_config`` files to include the script usage tests in runs of ``all_tests.py``. ``all_tests.py`` will now know how to find ``qiime_test_data``.