This should be referred to as *observation metadata*, and should support arbitrary metadata. Right now it looks like only the ```taxonomy``` key is supported. In the python code, we handle this by keeping a dictionary of metadata for each observation. Is it easy to mimic this in R?

```parseGreenGenesPrefix``` seems useful but I think it should be excluded (for now at least) as we'd like this functionality to match the python biom classes as closely as possible. Taxonomy strings currently aren't processed in any way when they're read in from file; they are simply stored with the rest of the arbitrary observation metadata.