Keep metadata

This pull request addresses Issue #34.   Metadata associated with an OTU table are now retained by predict_metagenomes.py and normalize_by_copy_number.py .   New transfer_observation_metadata and transfer_sample_metadata functions are introduced in predict_metagenomes.py and tested in test_predict_metagenomes.py.   A transfer_metadata wrapper is also included.  These functions are then called from normalize_by_copy_number.py.  In addition to the unit tests,   I also tested the results at the script level (normalize_by_copy_number.py has no library currently, which we may want to change later)...I'll send those files over the picrust-developers list since I don't want to add them to the repo and I'm not sure if I can attach them to the pull-request itself.

With this pull request integrated, we should be able to add metadata to the precalculated files and preserve it through the metagenome prediction process.