MetadataAdder, I choose you!

Converts add_metadata.py to use latest pyqi framework by introducing the :snake: **!!!!! ---- MetadataAdder ---- !!!!!!** :snake:. 

Also addresses the issues that I raised in @gregcaporaso's [pull request](https://github.com/biom-format/biom-format/pull/161) relating to print_biom_table_summary.py.

Unit tests currently fail because defaults are not filled in for Commands. Once @wasade's [pull request](https://github.com/bipy/pyqi/pull/125) gets merged, the unit tests should pass and can be added to.