Change PICRUSt precalculated file format from BIOM to in-house tab-delim format

New file format is output by default from predict_traits.py and are read by predict_metagenomes.py normalize_by_copy_number.py.

BIOM objects are still used internally and BIOM files can still be used with new options.

This helps address #55 since slow BIOM loading would make the new much larger precalc file not feasible. 

Note: the previous precalculated files have been converted to this new format and are temporarily available here for testing (once a new release is made they will be uploaded with it and added to a documentation page):
https://www.dropbox.com/s/qk9km7ihxszv53e/16S_18may2012_precalculated.tab.gz
https://www.dropbox.com/s/60j7xly3qavb7ft/cog_18may2012_precalculated.tab.gz
https://www.dropbox.com/s/fs19qjm51k2fk3j/ko_18may2012_precalculated.tab.gz