R: Added R-code directory, w/ installable "biom" package

The package itself should be submitted to CRAN when we think it is ready. It is already built-out enough and documented enough for testing. It contains some unit tests and examples. It lacks a vignette, any integration with trees (is that Newick-string idea incorporated into the file format yet? Examples of it?), and also lacks any package-internal data stored as RData binaries in the "data" top-level directory. 

All currently-included functions/classes are documented. 

Other features needed: 
- write-to-biom **Now supported**
- installation instructions in the absence of it sitting in an R repo or top-level GitHub repo **Now included**
- A vignette and example data were the two biggest criteria holding back submission to CRAN. **Now included**