Removed R-code subdir, and updated index.rst with links.

This contributes to [Issue
211](https://github.com/biom-format/biom-format/issues/211).

https://github.com/biom-format/biom-format/issues/211
added extra colons
Merge pull request #214 from joey711/rm-R-biom

Removed R-code subdir, and updated index.rst with links.