Add script level testing to all_tests.py

Most of the scripts (the important ones anyway) now have the 
appropriate test data in picrust_test_data directory (see #97). 

This PR adds the code to do the actual testing of the scripts. 
It uses QIIME code to do this, so this testing requires that QIIME to 
be installed, otherwise only module testing is performed. 

Currently all tests pass except for one in predict_metagenomes.py, but this
has already been fixed in PR #115 and is pending review.