Remove SparseDict and SparseMat backends

Also add hyperlinks to biom-format dependencies on index page.

Fixes #221. Fixes #205.
Update reference to biom.csmat
Merge pull request #224 from jrrideout/issue-221

Remove SparseDict and SparseMat backends
Remove python-code directory

Fixes #212.
Update setup.py and docs since python-code is gone
Merge pull request #225 from jrrideout/issue-212

Remove python-code directory