v0.3.9 speed (few 100X) improvement for sparse matrices

The `biom_data` parsing function now uses a vectorized (matrix-indexed)
assignment while parsing sparse matrices.
Unofficial benchmarks estimate a few 100X speedup.
Added tar.gz of 0.3.9 submitted to CRAN
Merge pull request #140 from joey711/master

R biom v0.3.9 speed (few 100X) improvement for biom_data() on sparse matrices