1.7.21 Import data.table. Faster, mem-efficient import usearch & legacy QIIME.

CHANGES IN VERSION 1.7.21
-------------------------

USER-VISIBLE CHANGES

- Updated `import_qiime` doc to emphasize it is intended for legacy
QIIME files.

- Much faster and mem-efficient import of legacy QIIME and usearch
files.

- Uses data.table syntax to better manage import of large files.

- Entire HMPv35 now imports in about 1 minute, low risk of mem-swap.

- Added dependency to data.table package

Also satisfies Issue 302, updated documentation about legacy QIIME files:
https://github.com/joey711/phyloseq/issues/302