replace summarize_otu_by_cat.py with collapse_samples.py

[gregcaporaso]

Initially discussed in #1796:

I had a thought on this this morning. My collapse_samples.py is as general, but has even more functionality that this script (can collapse based on combinations of fields, and has multiple options for combining the counts). Should we drop this in favor of that one? The only functionality that mine is lacking relative to this is the normalization, which could easily be added.