Move scale_metagenomes function to util.py

scale_metagenomes() can be used by other scripts,
so move it out of the scale_metagenomes.py script.
Add normalize options

After making metagenome predictions allow the abundances
to be "normalized" by dividing by the total sum of OTUs per sample,
(--normalize_by_otu), and/or by dividing by the total number of functions
per sample, (--normalize_by_otu).
Merge pull request #161 from mlangill/normalize

Add normalizing options to predict_metagenomes.py